Cessation Of Firing Research Articles

Haloperidol-induced suppression of carotid chemoreception in vitro.

Effects of antagonism of endogenous dopamine with haloperidol on single-unit frequency, interspike interval distribution, and interval serial dependency of the cat sinus nerve were tested using an in vitro carotid body-sinus nerve superfusion technique. A dose dependency of inhibition by haloperidol (0.05-2.0 microgram/ml) was observed. Superfusion with 1-2 microgram/ml haloperidol significantly reduced frequency within 5 min (P less than 0.05) and caused a complete cessation of firing within 25 min in 5 of 10 chemoreceptor units. Frequency recovered to control during drug washout. Acetylcholine (10-micrograms/ml superfusion or 500-micrograms bolus) increased sinus nerve activity under control conditions but not during superfusion with haloperidol. No effect of haloperidol on impulse serial dependency was detected. However, interval distribution was significantly altered by haloperidol in five of six chemoreceptor units. Our results suggest an excitatory role for dopamine in carotid chemoreception.

Immunological study of the rat hypothalamic ventromedial nucleus

Specific anti-sera were prepared by injecting the homogenates of rat ventromedial nucleus (VMH) and caudate nucleus (Cd) into the rabbit. Anti-VMH serum, after absorption of common components in rat normal serum and anti-Cd serum, reacted specifically to the rat VMH with only one precipitation line. Anti-VMH serum was successfully applied to 80 VMH neurons by electrophoresis through 5 barreled micropipettes. Fresh anti-VMH serum caused an irreversible response (increase and subsequent sudden cessation of firing) in 8 of the 25 VMH neurons tested. Anti-VMH serum reversibly inhibited 32 of 80 VMH neurons and 13 of these were also tested with glucose. Discharge rates of 12 of the 13 neurons increased by glucose. Most of these neurons were not affected by anti-Cd serum or normal rabbit serum. Results of these immunological and electrophysiological studies suggest the existence of specific membrane receptor binding sites on the glucoreceptor neurons in the VMH. These sites afford one route for producing the excitatory effect that glucose has on VMH neurons.

Detection and analysis of recurrent inhibition of firing motoneurons in man

During regular firing of "small" motor units, activated during weak voluntary contraction of the human soleus muscle, thick efferent fibers of n. tibialis were stimulated (a small M response was evoked, in which the small units did not participate). Peristimulus histograms of potentials of single motor units were constructed and the effect of stimulation on interspike interval duration was analyzed. The firing rate of the motor units was 4.5–7.6 spikes/sec. Stimulation of the nerve led to a sharp decrease in probability of their discharge or even complete temporary cessation of firing, i.e., it had a well marked inhibitory effect (lasting 10–20 msec). The latent period of inhibition (35–40 msec) was only a little longer than the latent period of the monosynaptic reflex of the soleus muscle. The effect of an inhibitory volley on duration of the interspike interval of the motor units depended on the time when the volley arrived during the interval. Lengthening of the interval was observed only if the inhibitory volley arrived in the second half or at the end of the interval. It is concluded that inhibition of firing of small motor units is due to Renshaw cells, activated on stimulation of axons of large motoneurons. The efficiency of a short (compared with the duration of the interspike interval) inhibitory volley reaching a motoneuron firing at low frequency characteristic of its adequate activation, is discussed.

Laser microbeam surgery: ultrastructural changes associated with neurite transection in culture

The exposure of neuronal and glial cell processes to a large number (up to 300) of 12-nsec laser pulses at a wavelength of 337 nm and energy densities below the threshold for nonlinear absorption results in a gradual, gentle process transection in the laser focus. Within 10 to 20 sec after cessation of firing, the process pinches in the target area. During this time, mitochondria become swollen and bleached, the plasma membrane develops an obvious tautness, microtubules disappear, and organelles accumulate to either side of the process constriction. Depending on the irradiation parameters, a local pinching may proceed to a transection in about 30 sec or it may reverse to yield a normal-appearing process in approximately 5 min. Severe process pinching is accompanied by a sudden depolarization that may last for 2 to 5 min and is usually followed by a repolarization to the original resting potential even if the process has transected. Spiral retraction of cut processes and cytoplasmic spillage observed after mechanical transections are not seen with this laser method. Process stretching is minimized or eliminated. Extensive vacuolization often associated with mechanical transections does not develop unless substrate involvement in the form of shock waves is apparent. For the performance of cell surgery in culture, this method appears to offer a reliable approach to morphological alteration of single cells and to the tailoring of two-dimensional neuronal networks. It should also allow more quantitative and better-controlled studies of axonotmesis, degeneration, and regeneration on the single cell level, and it may be used as a probe for the investigation of cytoskeletal dynamics. A mechanism describing the cytoskeletal changes associated with laser-induced cell process transection is proposed.

Location and spike-train characteristics of cells in anterodorsal pons having selective decreases in firing rate during desynchronized sleep.

Extracellular microelectrode recordings were made from single neurons in the pontomesencephalic brain stem of head-restrained but nonanesthetized cats whose behavioral state was continuously monitored polygraphically. A population of neurons was encountered with the unusual property of progressively decreasing discharge rate as the cats passed from waking (W) through synchronized sleep (S) to desynchronized sleep (D). In many cases there was a complete cessation of firing in desynchronized sleep, suggesting the designation D-off cell for this neuronal subtype. The D-off cell population was distributed in a bandlike zone across the brain stem at the pontomesencephalic junction with dense concentration of cells in aminergic nuclei: the serotonergic dorsal raphe and raphe linearis centralis and noradrenergic locus ceruleus and peribrachial regions. Aside from a sparse distribution in the central tegmental field, there were no D-off cells in pontine reticular nuclei where cells of the D-on type abound. The proportion of cells showing the D-off property and the magnitude of the state-related change were correlated across nuclear groups as follows: raphe and peribrachial (highest), locus ceruleus (intermediate), and reticular (lowest) for both values. The firing rates of D-off cells tended to be low in waking with very regular interspike intervals. This characteristic firing pattern and the anatomical distribution suggests that D-off cells may be tentatively identified as aminergic. The marked contrasts in aminergic neuronal activity between waking and desynchronized sleep are interpreted as possibly playing a role in state generation through progressive distribution of cholinergic neurons postsynaptic to the D-off cell population. Predictable results would not only be a change in the excitability of the postsynaptic population but also a change in response mode related to a shift in neurotransmitter throughout the brain.

Clipping Frequency and Fertilization Influence Herbage Yields and Crude Protein Content of 4 Grasses in South Texas

Crude protein content of herbage produced by buffelgrass, blue panicgrass, and Bell rhodesgrass was improved with nitrogen and phosphorus fertilization and clipping every 4 orJ weeks, compared to harvests only at the end of the growing season. Within a fertilization level, the 8-week clipping frequency generally increased dry matter production of the grasses over the 4-week clipping frequency or the end-of-season single harvest. Kleberg bluestem herbage generally contained less protein at all phenological stages than that of buffelgrass, blue panicgrass, or Bell rhodesgrass, and dry matter production was not increased by fertilization. Crude protein content of Kleberg bluestenm herbage was only slightly increased with the highest level of fertilization, regardless of clipping frequency. The South Texas Plains occupies nearly 9 million hectares of level to rolling terrain with elevations ranging from sea level to 95 m. Annual rainfall ranges from 45 to 75 cm with peaks occurring in May and September. Because of the long growing season and rainfall patterns, this area has a high potential for forage production. However, primarily as a result of continued excessive grazing of domestic livestock and cessation of fires, undesirable woody species have increase to the detriment of naturally occurring desirable vegetation (Scifres 1980). Brush management as a part of overall sound range management has proven its merit in restoring forage production (Scifres et al. 1976, 1980; Polk et al. 1976). Generally, natural revegetation through secondary succession is too slow for economical returns following high-cost mechanical control of undesirable woody species. Artificial revegetation with commercially available, adapted species is often considered the most feasible alternative for rapid restoration of these grazing lands. Major species that have been seeded successfully include buffelgrass (Cenchrus ciliaris), Bell rhodesgrass (Chloris gayana), Kleberg bluestem (Dicanthium annulatum), and blue panicgrass (Panicum antidotale). Although these grasses have been utilized in the overall range improvement program in south Texas for the past 4 decades, little is known about specific post-seeding management practices which could improve forage yield and quality. Brief descriptions, planting dates, seeding rates, and responses to irrigation and fertilization are found in experiment station leaflets (Novosad and Pratt n.d., Lancaster n.d., Lancaster 1949, and Anon. 1968). Since most of the livestock in this area are raised in yearlong cow-calf operations, high protein forage is required throughout the year to meet demands of growing calves and lactating cows. Waite At the time of this research, authors were graduate research assistant and associate professor, College of Agriculture,Texas Al and April 15 and 582 JOURNAL OF RANGE MANAGEMENT 36(5), September 1983 This content downloaded from 157.55.39.127 on Mon, 27 Jun 2016 05:36:06 UTC All use subject to http://about.jstor.org/terms Table 1. Fertility levels and pH of the soil of the study area where clipping frequency and fertilization was evaluated relatived to crude protein constants and herbage production of 4 grass species near Kingsville, Texas, during 1972-1973.' Nutrient availability

Livestock Grazing‐Fire Regime Interactions within Montane Forests of Zion National Park, Utah

Major differences were found between the vegetation structure of ponderosa pine—dominanted communities on the Horse Pasture Plateau and those on the nearby but isolated Church and Greatheart Mesas in Zion National Park. The Horse Pasture Plateau was heavily grazed by livestock in the late 19th and early 20th centuries, while the mesas were never grazed. Conditions on the mesas now approximate the pre—European situation of the region as described in the earliest written accounts. Pine, oak, and juniper sapling density and cover were much higher on the formerly grazed plateau than on the relict mesas. Herbaceous species dominated the groundlayer in mesa ponderosa pine savanna stands, while grass and forb cover was low on analogous sites of the plateau. Age—class distributions of major tree species further substantiated that major physiognomic changes have occurred on the plateau since the arrival of European man. Analysis of fire scars showed that prior to 1881, the mean fire—free interval for ponderosa pine stands on the plateau was 4 to 7 yr, while the interval for Church Mesa was 69 yr. Since there were no recorded fires on Church Mesa between 1892 and 1964, and yet no corresponding increase in sapling density, the increased understory density of plateau stands should not be attributed primarily to cessation of fires. Instead, heavy grazing by livestock and associated reduction of the herbaceous groundlayer promoted the establishment of less palatable tree and shrub seedlings, Fire, however, played an important secondary role in maintaining savanna and woodland communities.

Influence of electrical stimulation of the substantia nigra on spontaneous activity of raphe neurons in the anesthetized rat

Recent studies of afferent connections of the anterior raphe using the horseradish peroxidase technique have demonstrated a major projection originating in the substantia nigra (SN). The present acute electrophysiological study examined the influence of stimulation of this afferent on the activity of individual neurons in the raphe of the posterior midbrain and anterior pons ( n=51), and of a control group of cells ( n=15) located 2 mm lateral to the raphe. The predominant effect of SN stimulation at 0.1–1.0 mA, 1 Hz or 10 Hz, was suppression of raphe activity, with 63% of the cells showing cessation of firing following SN pulses and only 8% showing excitation. The average duration of suppression was 200 msec at 1 Hz and 38 msec at 10 Hz. In contrast, 40% of the lateral cells were excited, with 27% of the cells showing suppression. The mean duration of total suppression of lateral cell firing was 61 and 17 msec at 1 and 10 Hz, respectively. The results from the raphe cells are consistent with recent reports of stimulation of other forebrain and brainstem afferents to the raphe in which suppression of raphe activity was the main effect.

Effects of temperature on the discharges of muscle spindles and tendon organs.

1. In anaesthetized cats the effects of temperature on the nervous outflow from skeletal muscle via thick myelinated afferent fibres were studied. Single unit recordings were made from afferents of muscle spindles and tendon organs during slow and fast temperature changes of the medial gastrocnemius muscle which was deefferented by ventral root section and prestretched to a tension of 100 p. 2. Group I afferent units from muscle spindles were activated by warming and depressed by cooling, the effect of warming being much more pronounced than that of cooling. Afferents from secondary spindle endings with a high background discharge behaved similar to Ia fibres, whereas those with a low initial discharge rate showed an activation by cooling and a depression (mostly to cessation of firing) by warming. The discharges of group I afferents from tendon organs varied; an activation by warming was the most frequently observed reaction. 3. Some of the afferents from muscle spindles and tendon organs showed signs of a dynamic sensitivity to thermal stimulation, but in general the dynamic component in the responses to temperature changes was only small. 4. The results suggest that the afferent outflow via thick myelinated fibres from a resting, moderately prestretched muscle strongly depends on temperature. At raised intramuscular temperatures (about 42°C) the nervous outflow is characterized by an increased activity in all of the I a and many of the I b afferents, while the majority of group II spindle afferents will be depressed. In contrast, in a cold muscle (about 29°C) the nervous outflow via afferents from primary spindle endings will be reduced, while the net activity from secondary spindle endings will be increased and no marked changes are expected to occur in the discharges of I b fibres.

Neuronal firing patterns during generalized penicillin epilepsy in the awake cat

Single unit activity was recorded in the neocortex of awake cats during spontaneous spike wave discharges. Ongoing activity was interrupted during paroxysmal events and was replaced by 1. (1) bursts of action potentials coincident with the EEG spike and cessation of firing during the wave, or 2. (2) cessation of firing for the duration of the paroxysm. Other units which displayed little or no background activity were recruited to fire during the EEG spike. Surface cerebellar stimulation at high frequencies led to decreased neuronal activity while single pulse shocks resulted in short latency activation. These results were compared to studies of spike wave activity elicited by electrical stimulation.

Discharge patterns of the nucleus parabrachialis lateralis neurons of the cat during sleep and waking

Discharge patterns of the nucleus parabrachialis lateralis (PbL) neurons located in the dorsolateral pontine tegmentum were investigated in 7 unrestrained, freely behaving cats with a multiwire electrode bundle method. Among 84 PbL units recorded so far, 31% of the units exhibited a marked reduction or complete cessation of firing during paradoxical sleep (PS) (PS-off cells), while 55% of the units exhibited an increase of firing during PS (PS-on cells) as compared with quiet wakefulness (QW) and slow wave sleep (SWS). The remaining 14% of the PbL units exhibited no remarkable change of firing rate and pattern during the sleep-walking cycle. The PbL PS-off cells were further characterized by a slow and regular firing pattern during QW and SWS, and they were similar in several respects to the PS-off cells reported in the nucleus raphe dorsalis and locus coeruleus complex. Six of the PbL PS-on cells were tightly phase-locked with PGO waves, and the spikes preceded by 5-25 msec the onset of the PGO waves recorded in the lateral geniculate nucleus. Seven other PbL PS-on cells were related to the locomotor activity observed in active wakefulness. In light of the present results, the functional significance of the PbL units has been discussed.

Postinhibitory unit response of crustacean stretch receptors after direct and recurrent inhibition

The postinhibitory response of a slowly adapting neuron was investigated in experiments on an isolated preparation of crustacean stretch receptor and abdominal nerve chain. The structural features of this preparation are such that this response can be regarded as the response of the postsynaptic membrane to synaptic inhibition and not the action of synaptic excitation. IPSPs arise in the slowly adapting neuron in response to stimulation of the abdominal nerve chain (direct inhibition) or to excitation of the neuron itself (recurrent inhibition). The postinhibitory response consists of the development of action potentials or an increase in their amplitude and frequency. The magnitude of the response is determined by the duration of the inhibition and the state of the neuron membrane. The postinhibitory response was strongest when IPSPs were superposed on cathodal depression. IPSPs and an intracellular hyperpolarizing current evoke similar postinhibitory responses. Repetitive excitation of an inhibitory neuron may result in the appearance of a regular spike discharge from a previously inactive neuron through the mechanism of the postinhibitory response. Activation of a chain of recurrent inhibition increases the duration of the postinhibitory response evoked by direct inhibition or by a hyperpolarizing current. The existence of a chain of recurrent inhibition prevents the cessation of firing by a neuron during increasing cathodal depression. A mechanism of postinhibitory rebound lies at the basis of this phenomenon.

Carotid sinus baroreceptor responses to recurrent trapezoidal pressure pulses

Recurrent trapezoidal pressure pulses were applied to the carotid sinus of cats while recording from single fibers. Steady-state responses to these recurrent pressure pulses were studied. The salient features of the response were (i) a near linear rise to a dynamic peak overshoot, (ii) an adaptive fall from the peak with two distinct phases, an early rapid fall followed by a slow decay and (iii) immediate cessation of firing at the onset of the falling portion of the trapezoidal pressure pulse for all pressure velocities tested, i.e., dp dt ranging from 375 mm Hg/sec to 1850 mm Hg/sec. For positive pressure velocities, the slope of the linear rise of the steady-state response was found to be a power function of the pressure velocity, i. e., slope = K (pressure velocity) α. It was observed that α and K varied with the size of the pressure step and its initial level. The effect was such that as the magnitude of the pressure step increased, the slope decreased. These results indicate that the baroreceptor has the property of adjusting itself to steady input conditions.

Multivariate Analysis of Radial Growth of Black Cherry (Prunus serotina Erhr.) in Southern Wisconsin Oak Forests

Following extensive disturbance of oak forests in southern Wisconsin in the 1930 decade, black cherry, Prunus serotina Erhr., successfully established across a wide range of habitats. The purpose of this study was to assess its ability to reach maturity on all parts of the moisture gradient. Radial growth in 122 black cherry saplings and trees was correlated with edaphic and micrometeorological parameters during 3 growth seasons. Although total basal area change and the duration of rapid growth increased consistently from suppressed through the dominant crown class, the metabolic potential was distinctly lower in the suppressed crown class. The intermediate crown class was very similar in metabolic potential to codominant and dominant classes. Correlation of growth rate to environmental parameters was complicated by marked within-season changes. Correlation between soil moisture and growth rate in 1965 showed a consistent monthly increase from May through August (r = .19*, -.27**, .34**, .38**, respectively*,** = statistical significance at .05 and .01 probability levels) but decreased in September (r = .14). Rapid soil moisture depletion in June coincided closely with the period of maximum growth acceleration. The growing seasons of 1964, 1965, and 1966 contrasted sharply in temperature and soil moisture levels. The Palmer severity index indicated that drought in 1964 was greater than in 1934. May-September air temperature and precipitation in 1965 departed only slightly, from normal while in 1966 air temperature was low. Analysis of growth response by principal components analysis and multiple regression clearly reflected these differences. Despite low metabolic potential and notable dependence on soil moisture levels, prediction of survival in suppressed strata was complicated by the marked capacity of black cherry to regenerate by means of basal sprouts and the eventuality of disturbance in the oak canopy. INTRODUCTION The purpose of this study is to determine significant factors controlling radial growth of black cherry (Prunus serotina Erhr.) in southern Wisconsin oak forests. In this area, oak forests originated with the cessation of fires which maintained prairie and oak savanna in the era predating Europ;ean settlement 125 years ago (Curtis, 1959). Black cherry is presently the most abundant species in sapling and small tree sizes but is relatively uncommon as a canopy tree (Curtis, 1959; Cottam, 1963). Its widespread establishment is the result of ubiquitous disturbances in the oak forest and its marked capacity for opportunistic behavior (Auclair and Cottam, 1971). Climax species such as Acer saccharum and Tilia americana with 1 Present address: Biology Department, McGill University, Montreal 101, Quebec, Canada.

The influence of microelectrophoretically applied biogenic amines, cholinomimetics and procaine on synaptic excitation in the corpus striatum.

Abstract The effect of microelectrophoretically administered biogenic amines, cholinomimetic drugs and procaine upon synaptically induced discharge activity of neurones in the corpus striatum is investigated in unanaesthetized rabbits. This investigation is based on previous studies of the effect of excitatory and inhibitory amino acids upon the neurones in the corpus striatum following thalamic stimulation. 1. Dopamine (DA) inhibited spontaneous and glutamate-induced, as well as synaptically-induced, firing of the majority of neurones in the caudate nucleus and in the putamen. A mutual dopamine-glutamate antagonism was demonstrated. Submaximal effective doses of DA induced considerable increase of the latency of the first discharge of the primary activation. In some cases cessation of firing following application of high glutamate dosages, (due to hyperdepolarization) could be reversed by application of DA. 2. Noradrenaline (NA) and 5-Hydroxytryptamine (5-HT) inhibited the discharge activity in most neurones. Among the cholinomimetic drugs investigated, oxotremorine showed the most distinct inhibitory effects. With acetylcholine (ACh) both, inhibitory and excitatory effects were seen, the former being more frequent, however. 3. The inhibitory effects of a local anaesthetic agent, procaine, showed remarkable differences compared with those of DA: Lack of a significant increase in latency of the first discharge of the primary activation following application of submaximal procaine dosages, reduction of the amplitude of spikes, slow onset and slow reversal as well as lack of reversibility of the procaine-induced inhibition by glutamate. 4. The similarity between the DA-effect and cationic (hyperpolarizing) current, the antagonistic effect of glutamate and anionic (depolarizing) current, the differences compared to procaine-induced inhibition, seem to indicate that in the inhibitory effect of DA repolarization (or hyperpolarization) of the cell membrane is involved.

B a2+ and S r2+ reversal of the inhibition produced by ouabain and local anesthetics on membrane potentials of cultured heart cells

Ouabain (10 −6−10 −3 M) produced only inhibitory effects on the electrical activity of cultured chick heart cells; partial depolarization and depression of the action potential rate of rise gradually occurred leading to eventual cessation of firing. Cocaine and tetracaine produced depolarization and inhibitory effects similar to those of ouabain. The depressed electrical activity and quiescence induced by these drugs was rapidly reversed by Sr 2+ or Ba 2+ (5–10 m M), which usually produced a large hyperpolarization before the onset of the reversal of inhibition. As previously reported, Ba 2+ and Sr 2+ (5–10 m M) rapidly converted quiescent nonpacemaker cells into pacemakers; Ba 2+ increased membrane resistance (presumably by decreasing K + conductance) and produced partial depolarization, whereas Sr 2+ always hyperpolarized. With lowering of [Na +] 0 below 50 m M using Li + or sucrose replacement, the action potential rate of rise became progressively depressed, the overshoot usually diminished, and firing eventually ceased, whereas the resting potential was only slightly affected; thus the inward current during an action potential seems to be normally carried by Na +. The lack of a hyperexcitable effect of ouabain is consistent with the hypothesis that catecholamine release is necessary for such action. The depolarization produced by ouabain and the local anesthetics may be due to inhibition of active ion transport. The rapid hyperpolarization produced by Ba 2+ and Sr 2+ in the presence of ouabain, cocaine and tetracaine could be due to a change in conductance or to stimulation of the Na + pump.

Response of muscle spindles to stimulation of the sympathetic trunk

Examination was made for influence of sympathetic innervation on the discharge of receptors of otherwise de-efferented muscle. Firing of units in the cat's gastrocnemius was monitored in filaments of dorsal roots while repetitive electrical stimuli were given to the distal stump of one lumbar sympathetic trunk. A characteristic response, consisting of a delayed and gradual acceleration followed by more or less abrupt cessation of firing, was found from both spindle and tendon organ afferents. Blood flow through the leg was markedly reduced during stimulation. This observation and the failure of the full effect to appear when vasoconstriction was blocked, as well as the similarity of the effect to that obtained on the same afferent during compression of the arterial supply, suggest that the response arose mainly as the indirect result of vasoconstriction. Masked by the full response, there may also be a lesser acceleratory effect which was revealed by sympathetic stimulation in the cat under the influence of the adrenergic blocking agent phenoxybenzamine and which was reproduced in some animals by small intra-arterial injections of epinephrine. Slowness in onset of this acceleration following sympathetic stimulation suggests that humoral sensitization of the peripheral endings was the cause, rather than direct motor innervation of intrafusal fibers.

Transmembrane potentials and contractility in the pregnant rat uterus.

Surviving segments from the uteri of pregnant Long-Evans rats were studied. The rats were within 12 hours prepartum. Spontaneous contractility was measured by use of a strain gauge transducer. By means of the microelectrode technique of intracellular recording, transmembrane potentials were observed simultaneously. It was observed that trains of transmembrane action potentials appeared during rhythmic contraction. The frequency was low at the onset, increased to a maximum averaging greater than one per second and then diminished before cessation of firing. The average duration of impulse trains was 35 seconds at 30°C, less than the duration of the total contraction. Slow depolarization between action potentials often appeared after the first several cycles in a train of impulses. This phenomenon resembled the cardiac pacemaker prepotential. In light of similar experimental observations on stretch receptors by other investigators, it was considered possible that interfiber spread of excitation in uterus might be mediated by stretch.

Shrub Invasion of Southern Arizona Desert Grassland

E NCROACHMENT of mesquite, burroweed, and other noxious shrubs has seriously reduced the carrying capacity of thousands of acres of desert grassland ranges. Two principal theories have been advanced for this invasion-grazing and control of fires. In the early part of the century Griffiths (1910), and Thornber (1910) observed the large mesquites along the water courses, and the presence of young mesquites on the ridges separating them. Griffiths predicted that both mesquite and other shrubs would increase in size and number on these ridges. Both expressed the opinion that fire, formerly unchecked and with abundant fuel, had maintained the grassland in its original shrub-free state. They believed that livestock grazing had controlled fires indirectly through removal of fuel by trampling and grazing. Later, Wooton (1916) observed that Griffiths' prediction was coming true. He noted that occasional fires were the only check on the increasing shrubs. Leopold (1925) advanced this same theory for the short-grass areas of northern Arizona and called attention to the encroachment of oak and juniper from the higher elevations. This invasion followed the cessation of fires which was shown by burn scars to have been some 40 years earlier. He doubted, however, that the lower levels of the desert grassland ever supported sufficient vegetation for widespread or frequent fires. Humphrey (1949) called attention to the earlier work of Thornber, Griffiths and Wooton. In a study of two burned areas near Tucson, Arizona, he observed that burroweed and snakeweed were effectively controlled by fire. He also recorded a moderate degree of control of such shrubs as jumping cholla; mesquite and palo verde. The grazing theory also has many adherents (Allred 1948, Upson et al 1937, Young et al 1948). In general, they believe that breakdown of the sod and compaction of the soil surface through grazing pressure and drought has led to the invasion of shrubs, causing a grazing disclimax. In refutation of the fire theory of shrub control, Allred (1948) has pointed out that some of the most frequently recurring fires are in the Texas brush country where mesquite is densest. It should be kept in mind that Allred's observations pertain primarily to Texas while the workers referred to in the previouis paragraphs obtained their results in Arizona. Further support is lent to the grazing theory by the fire-resistant nature of mature mesquites. Although the top is readily killed by hot grass fires, the plant is a stump sprouter, and usually sprouts following severe injury to the top. Many other desert plants are similar to mesquite in this respect. Neither of these theories has been adequately proved. Doubtless both fire and grazing have played an important part in the ecology of the desert grassland. Further information concerning the rate of shrub invasion, its relation to management practices and forage production, and the successional relationships of the invaders is necessary for control of the