Young Primordia Research Articles

Leaf Development in Trifolium repens

Leaf development is described quantitatively, with particular reference to rates and durations of cell division and cell expansion in the various tissues of the lamina and to development of the vascular system. From these data the following three hypotheses are suggested and are discussed in relation to the diversity in leaf structure between species and between tissues: (1) Relative rate of leaf expansion is highest in the young primordium and then tends to decline until maturity. (2) Relative rate of cell division is also highest in the young primordium and then declines; but it does not necessarily decline at the same rate as that of leaf expansion and declines at different rates in different tissues. (3) Cell size depends on the proportional rates of leaf expansion and cell division.

Capacity of Fragments of Leaf Primordia to produce Whole Leaves

FOR a number of years both microsurgical and sterile culture techniques have been brought to bear on problems of leaf development in the vascular plants. Among the phenomena which have been investigated is the elusive process of leaf determination whereby the pattern of leaf morphogenesis is imposed on tiny outgrowths or primordia on the flanks of the apical meristem. Partial isolation of younger primordia by variously placed vertical incisions has led, in some cases, to the development of centric leaves, that is, organs which are determinate in their growth, but radial rather than dorsiventral in their symmetry1.

Der Einfluss des Lichtes auf die Bildung von Blattprimordien am Vegetationskegel der Keimlinge vonSinapis alba L

Seedlings of white seeded mustard (Sinapis alba L.) have been investigated with respect to the influence of light (red, far-red, white) on the rate of formation of primordia at the apex. The results show that the formation of primordia is strongly increased by light and that this control is exerted mainlyvia the phytochrome system and the “high energy reaction” )=blue, far-red reaction) of photomorphogenesis (cf.Mohr 1962). Apparently photosynthesis can also be important in the case that the supply of organic material limits the rate of those growth processes which have been induced by the photomorphogenetically effective radiation. The morphological appearance of the stem apex and of the youngest primordia is the same in light and dark. It is concluded that light increases the rate of primordia formation by promoting the mitotic activity in the apical meristem. Growth of the primary leaves of the mustard seedling is also greatly promoted by light. Both the phytochrome system and the “high energy reaction system” are involved. Experiments indicate that this control by light is exerted in a way similar to the light control of the growth of the cotyledons (Mohr 1959b).

THE DEVELOPMENTAL MORPHOLOGY OF NICOTIANA TABACUM ‘WHITE BURLEY’ AS INFLUENCED BY VIRUS INFECTION AND GIBBERELLIC ACID

Stein, Diana B. (U. Montana, Missoula.) The developmental morphology of Nicotiana tabacum ‘White Burley’ as influenced by virus infection and gibberellic acid. Amer. Jour. Bot. 49(5): 437–443. Illus. 1962.—‘White Burley’ tobacco with Severe Etch Virus (SEV) displayed a reduction of plant height which was overcome to a limited degree by spraying with gibberellic acid (GA). Spraying with GA, while hastening maturity in terms of earlier elongation and flowering, also prolonged the life of treated plants. Infection with SEV caused an increased rate of leaf production, and since flowering was also delayed, the greater number of leaves is produced by infected plants. Spraying with GA also increased the rate of leaf production but did not increase the final number of leaves produced. All groups except the unsprayed virus‐infected plants showed a spurt in leaf production just prior to flowering. The pattern of internode elongation was obtained by the periodic measurement of individual internodes. In general, this pattern reflected the stunting properties of the virus and the growth‐promoting properties of the GA. Internodes which were mature at time of spraying with GA were not affected. Infection with virus generally delayed elongation and shifted the internode pattern. Infection with SEV tended to reduce the size of leaves already present prior to inoculation, but some leaves produced after infection were actually 1arger than the same leaves on the controls. Spraying a healthy plant with GA made older leaves longer and wider, while less mature leaves at time of treatment tended to be longer and narrower. Spraying with GA reversed the reduction in size caused by the virus only if the leaf was a very young primordium or was formed during the course of treatment with GA.

GROWTH PATTERNS OF PEA SEEDLINGS IN DARKNESS AND IN RED AND WHITE LIGHT

Thomson, Betty F., and Pauline Monz Miller. (Connecticut Coll., New London.) Growth patterns of pea seedlings in darkness and in red and white light. Amer. Jour. Bot. 48(3): 256–261. Illus. 1961.—Seedlings of peas were grown in vermiculite at 22°C. and exposed 16 hr. daily to red or white light or kept in darkness. Others were grown in soil in the greenhouse. Samples harvested daily to 16 days were dissected, the length of each internode and leaf measured and the total number of leaves and leaf primordia counted. The form of the stem apex and youngest primordia and interrodes is the same in light as in darkness. Leaf production is accelerated very slightly and the growth of leaves and internodes is decidedly accelerated by light. Leaf‐leaf, leaf‐internode and internode‐internode correlations indicate that the morphogenetic effect of light is limited to later stages of organ growth. Dry weight is consumed more rapidly in light than in darkness, probably because of more rapid growth and slightly greater amounts of respiring tissue in light‐grown plants.

Radiation-Induced Genetic Markers in the Study of Leaf Growth in Zea

Stein, Otto L. (Montana State U., Missoula.), and Dale Steffensen. Radiation-induced genetic markers in the study of leaf growth in Zea. Amer. Jour. Bot. 46(7): 485–489. Illus. 1959.—Corn embryos heterozygous for the plant color mutant yellow-green were treated with moderate doses of radiation. The sectors resulting from the loss of the dominant gene Yg2 furnish a graphic illustration of the growth of an organ in space and time. Wet embryos were irradiated and leaves 3 through 6 were analyzed. The total number of sectors per leaf increases in relation to the amount of leaf tissue present at time of irradiation and their distribution varies, indicating a change in mitotic activity during early ontogeny of the leaf. As represented by leaf 6 in the mature embryo, cell division is rather uniform throughout the young primordium. Soon thereafter (leaf 5) cell divisions in the apical region increase, though cell lineages remain short. In the basal region of the leaf, the proportion of cell division decreases but those cells which do divide continue their activity over a longer period than the apical cells. Longer files of mutant cells are formed in such a way. The mid regions of the leaf maintain their status quo in terms of distribution of sectors. The results indicate that differentiation of the intercalary meristem as a thin band occurs early in the development of the leaf primordium. The use of genetic techniques in conjunction with radiation, in this case as a “timed” release of genetic markers, may be a useful tool in the precise analysis of the dynamic aspects of organogenesis and histogenesis.

RADIATION‐INDUCED GENETIC MARKERS IN THE STUDY OF LEAF GROWTH IN ZEA

Stein, Otto L. (Montana State U., Missoula.), and Dale Steffensen. Radiation‐induced genetic markers in the study of leaf growth in Zea. Amer. Jour. Bot. 46(7): 485–489. Illus. 1959.—Corn embryos heterozygous for the plant color mutant yellow‐green were treated with moderate doses of radiation. The sectors resulting from the loss of the dominant gene Yg2 furnish a graphic illustration of the growth of an organ in space and time. Wet embryos were irradiated and leaves 3 through 6 were analyzed. The total number of sectors per leaf increases in relation to the amount of leaf tissue present at time of irradiation and their distribution varies, indicating a change in mitotic activity during early ontogeny of the leaf. As represented by leaf 6 in the mature embryo, cell division is rather uniform throughout the young primordium. Soon thereafter (leaf 5) cell divisions in the apical region increase, though cell lineages remain short. In the basal region of the leaf, the proportion of cell division decreases but those cells which do divide continue their activity over a longer period than the apical cells. Longer files of mutant cells are formed in such a way. The mid regions of the leaf maintain their status quo in terms of distribution of sectors. The results indicate that differentiation of the intercalary meristem as a thin band occurs early in the development of the leaf primordium. The use of genetic techniques in conjunction with radiation, in this case as a “timed” release of genetic markers, may be a useful tool in the precise analysis of the dynamic aspects of organogenesis and histogenesis.

RADIATION DAMAGE IN SHOOT APICES OF CONCORD GRAPE

Pratt, Charlotte. (N. Y. State Agri. Expt. Sta., Geneva.) Radiation damage in shoot apices of Concord Grape. Amer. Jour. Bot. 46(2) : 103‐109. Illus. 1959.—Dormant rooted cuttings of ‘Concord’ grape were exposed either to 200‐6000 r of X rays or to 2‐6 hr. of thermal neutrons (average total dose of 4.80 × 1011 to 1:59 × 1013 neutrons/cm.2). Percentage of plants surviving in the field tended to decrease with dosage beginning with 2000 r of X rays or 2 hr. of thremal neutrons. It showed a sharp drop in lots which had received doses of 4000 r of X rays or 4 hr. of thermal neutrons. Frequency of hudless shoots (shoots lacking a terminal bud after the first leaves emerged) increased with dosage, beginning with groups which had received 2000 r of X rays or 2 hr. of thermal neutrons. Microscopic examination of radiation‐damaged shoot apices showed one or more of the following characteristics: (1) shape and organization of the apical meristem different from that of the untreated shoot apex, which is described; (2) no young primordia; (3) acropetal advance of maturation of cells; in 4 cases cells with spirally thickened, lignified walls were found 1‐4 cells deep in the meristem; (4) collapse of cells in the tunica and corpus; formation of periderm and deeper, cambium‐like divisions below the wound; (5) site of the former apical meristem either carried up on the base of a single terminal leaf or remaining a flat area between terminal leaves. Aspects of radiation sensitivity of the shoot and theories of shoot organization with reference to natural and induced chimeras are discussed.

Mitotic Activity during the Early Differentiation of the Scleral Bones in the Chick

ABSTRACT The dispositions of mitoses in the mesenchyme beneath the conjunctiva of parts of a 7-, 8-, 9-, and 10-day embryo have been ascertained. For the purpose of analysis, the number of mitoses, cell population density, and mitotic index have been found in small segments of annuli which include parts of the rings of bone primordia, and the mesenchyme between them. Twenty-three primordia were studied. It is found that the cell population density increases from about 4−14 × 105 per cubic mm within the primordia, but only from about 4−9 × 105 in the mesenchyme between them. Mitotic activity increases sharply within young primordia from about 12-27 mitoses per 1,000 cells, and thereafter decreases. It is found that this mitotic activity probably occurs in three waves of decreasing intensity. Between the primordia mitotic activity is at a markedly lower level and probably follows a simpler pattern. The cells involved in this mitotic activity in the primordia are particularly those just beneath the conjunctival papilla, and the number of mitoses decreases the greater the distance from the conjunctiva. In the 8-and 9-day embryos, the number of mitoses is inversely proportional to the distance. Calculation indicates that in each of the three waves of cell division, the cells that divide do so only once; and that this mitotic activity is adequate to account for the increase in cell population density which is found. The cause of the increase in the number of cell divisions in the primordia is not clear, but the result is probably the production of blastemas, the cells of which become the osteoblasts of the scleral bones.

Further experimental observations on the shoot apex ofDryopteris aristataDruce

Further experimental observations on the shoot apex of<i>Dryopteris aristata</i>Druce

Experimental and Analytical Studies of Pteridophytes

IN our this knowledge paper experimental of phyllotaxis observations in ferns; but are a full set consideration out which contribute of this comto our knowledg of phyl taxi in ferns but a full consideration of this omplex subject must be reserved until later. These observations relate to the induction of tangential tensile stress in the shoot apex as a result of the growth of the young leaf-primordia. The view is advanced that such stresses constitute one of the factors which determine the position of new leaf-primordia. During the formation of the individual leaf-primordium in a fern such as Dryopteris aristata, a conspicuous enlargement of the basal region takes place, particularly in the tangential direction. This is mainly due to the development of parenchyma in the pith and cortex. In a very young primordium, as seen in transverse section, the incipient vascular tissue consists of an uninterrupted crescent of small-celled tissue. The growth of this tissue does not keep pace with the growth of the pith, and it becorties broken up into a number of separate strands or meristeles, i.e. apparently as a result of the tangential stress imposed by the pith or jointly by the pith and cortex (Wardlaw, 1944, 1945). The incipient vascular tissue in the regions of disruption undergoes a parenchymatous development; in other words, potential phloem and xylem are transformed into parenchyma. Experimental evidence of this phenomenon has also been obtained (Wardlaw, 1947). In relation to these developments in the leaf-primordia, the incipient shoot-stele, which initially consists of an uninterrupted cylinder, is also subjected to tensile stress in the regions of insertion of the leaf-traces, and parenchymatous leaf-gaps develop. The adult stele thus consists of an open vascular meshwork or dictyostele. These conceptions of stelar morphology in ferns were based on anatomical studies of development at the shoot apex. Experimental proof that leaf-gaps are due to the enlargement of the leaf-bases was sought by destroying leaf-primordia at a very early stage: by this procedure the development of local stresses would be avoided and the vascular cylinder would remain uninterrupted, i.e. solenostelic. When the appropriate experiment was carried out the result predicted was obtained (Wardlaw, 1944). Such observations suggest that at the apical meristem of ferns a conical [Annals of Botany, N.S. Vol. XII, No. 46, April, 1948.] 966.46 H

SHOOT APEX IN GRASSES AND CEREALS

THE lack of readily available illustrations and 1literature on the morphology of the shoot apex in the Gramineae, coupled with the fascination to be obtained from actually viewing the living growing point and the newly initiated primordia in various stages of their early development, prompt a note for the benefit of other workers. Using a mounted needle under a fixed lens (or for preference, a dissecting-microseope) a little care will enable the outer leaves of most grass shoots to be removed one by one until the very young primordia and the growing point are revealed. Of the grasses so far studied, Glyceria fluitans R.Br. is by far the easiest to dissect, since its leaves are conduplicate and possess little sclerenchyma, etc., so that the tissues cut exceptionally well and yield to the needle in a manner curiously reminiscent of snow.

Shape Changes During Fruit Development in Cucurbita and their Importance in the Study of Shape Inheritance

(1) During fruit development from early ovary primordium to maturity, in disk fruited and sphere fruited plants of Cucurbita pepo, there is a slight progressive increase in degree of flattening, or ratio of equatorial diameter to polar diameter. (2) Fruits will stop growth and become mature at widely different sizes if environmental conditions vary or if there is segregation for size factors. The mature fruit retains the shape index characteristic of the developmental stage which it had attained when growth ceased. Thus the segregation of genetic factors controlling shape is rendered much less distinct if there are size differences in the population. (3) By determining shape indices for small ovary primordia instead of for mature fruit, a population segregating for shape may be studied at a uniform size and the variability due to size differences may be eliminated. This method has the added advantages of making possible a much larger number of determinations for each plant, and of eliminating minor shape differences which often appear during development. (4) In three F2 pedigrees showing monofactorial segregation for disk and sphere fruit shape, a comparison of the plotted indices of young primordia, 1 to 8 cc in volume, with those of mature fruits from the same plant showed in every case a much sharper segregation for shape in the primordia than in the fruits. (5) It is believed that this method of studying earlier stages in development will make possible a more accurate analysis of shape inheritance in cases where the genetic situation is more complex.

Origin of Adventitious Roots in Coleus Cuttings

1. Adventitious roots arising between the fibrovascular bundles from the bases of young cuttings of Coleus blumei originate in one to several adjacent cells of the pericycle. 2. The first division of the initial cells of a root is usually tangential, but it may be radial, or even oblique. 3. During the early stages of development the daughter cells divide without first increasing in size, so that a young primordium consisting of many cells may occupy the same space as the pericyclic cells from which it originated. 4. The primordium enlarges and bulges into the cortex before differentiating into the tissues of a root. 5. Subsequent development is precisely as described by the early investigators of the subject of root development.