Yucca Whipplei Research Articles

THE RELATIONSHIP BETWEEN THE PRIMARY THICKENING MERISTEM AND THE SECONDARY THICKENING MERISTEM IN YUCCA WHIPPLEI TORR. II. ONTOGENETIC RELATIONSHIP WITHIN THE VEGETATIVE STEM

Histological and anatomical observations were made on plants ranging from embryos to 1‐yr‐old plants of Yucca whipplei Torr. ssp. percursa Haines. Our objective was to determine the time and position of origin, ontogenetic relationship, and function of the primary thickening meristem (PTM) and the secondary thickening meristem (STM) during development of the vegetative axis. The PTM arises in the stem periphery during germination. It is longitudinally continuous from beneath the youngest leaf primordia to the primary root, and functions in the production of primary vascular bundles and ground tissue. The STM arises ontogenetically from the PTM in the base of 2‐ to 3‐month seedlings and produces secondary vascular bundles and ground tissue. The PTM and STM, primary and secondary vascular bundles, and files of primary and secondary ground tissue are continuous at all developmental stages studied. Development of the primary body is histologically and quantitatively similar to development in monocotyledons which possess only a PTM; and secondary growth is interpreted as a developmental continuation of the process of primary thickening.

The Relationship Between the Primary Thickening Meristem and the Secondary Thickening Meristem in Yucca whipplei Torr. II. Ontogenetic Relationship within the Vegetative Stem

Histological and anatomical observations were made on plants ranging from embryos to 1-yr-old plants of Yucca whipplei Torr. ssp. percursa Haines. Our objective was to determine the time and position of origin, ontogenetic relationship, and function of the primary thickening meristem (PTM) and the secondary thickening meristem (STM) during development of the vegetative axis. The PTM arises in the stem periphery during germination. It is longitudinally continuous from beneath the youngest leaf primordia to the primary root, and functions in the production of primary vascular bundles and ground tissue. The STM arises ontogenetically from the PTM in the base of 2- to 3-month seedlings and produces secondary vascular bundles and ground tissue. The PTM and STM, primary and secondary vascular bundles, and files of primary and secondary ground tissue are continuous at all developmental stages studied. Development of the primary body is histologically and quantitatively similar to development in monocotyledons which possess only a PTM; and secondary growth is interpreted as a developmental continuation of the process of primary thickening.

Spatial and temporal dispersion patterns of pollinators and their relationship to the flowering strategy of Yucca whipplei (Agavaceae).

A field investigation of the mutualistic interaction between a monocarpic perennial plant, Yucca whipplei, and its host-specific pollinator and seed predator, Tegeticula maculata (Lepidoptera: Prodoxidae), was conducted to determine how the resource utilization pattern and population dynamics of the pollinator have influenced the evolution of the flowering and fruiting pattern of the plant. Although the temporal pattern of emergence of pollinators results in a relatively close tracking of flower abundance within a season, the ratio of pollinators to open flowers does vary significantly within a season, as well as between seasons. At any point in time during the flowering season, the population of adult yucca moths is distributed evenly among the available flowers, so that the number of pollinators on an inflorescence is directly proportional to the number of open flowers available. The relative isolation of individual flowering plants appears to have little effect on the distribution of pollinators among inflorescences. The number of fruits initiated on a plant is directly proportional to the number of flowers produced, and is also partially determined by the time of flowering. Yucca whipplei always produces many more flowers than fruits. Most flowers are not fertilized, and the plants also generally abort and abscise immature fruits after flowering. Fruit production of at least some plants, however, appeared limited by pollination. It is also expected that in some years the relative abundance of pollinators will be low enough that most plants will be pollinator-limited. It is suggested that the pattern of flowering and fruiting of this species has evolved in response to the unpredictability of pollinator availability, both within and between seasons. Resource uncertainty and selection acting on the male component of fitness may also be involved.

Regulation of Flower, Fruit and Seed Production by a Monocarpic Perennial, Yucca Whipplei

(1) Flower, fruit and seed production of the monocarpic perennial, Yucca whipplei, were measured in a population in southern California. The size of the rosette, number of flowers reaching anthesis, number of fruits initiated, number of fruits aborted, and number of fruits matured were determined for all plants flowering at two sites in 1979 and one site in 1980. Estimates of the average number of ovules per capsule and average seed weight were also obtained for plants flowering in 1979. (2) The number of flowers reaching anthesis was directly proportional to the size of the rosette. The number of fruits initiated on a plant was much less than, but proportional to, the number of flowers produced. (3) Many fruits were aborted and abscised prematurely in both years. Mature fruit production was determined by both the size of the rosette (or inflorescence) and the number of fruits initiated. Mature fruit set (mature fruits/flowers) averaged 9. 1% in 1979 and 10.0% in 1980. (4) The number of ovules per capsule did not vary with plant size; however, average seed weight was positively correlated with rosette size at one site, but not at the other. (5) Yucca whipplei regulates its maternal expenditure at the stages of flower determination and fruit maturation. Unpredictable pollinator availability both within and between seasons appears to favour this pattern of reproduction. (6) A major difference in rainfall between the two years of the study had no significant effect on percentage mature fruit set. The plants may respond to changes in moisture availability by adjusting the size of the inflorescence rather than the proportion of flowers maturing to fruits.

Fruit abortion and the regulation of fruit number in Yucca whipplei

Flowering and fruiting patterns of Yucca whipplei (Agavaceae), a monocarpic perennial of the southern California chaparral, were studied to determine the role of fruit abortion in the regulation of seed yield. Data on reproductive expenditure, fruit abortion and fruit maturation were obtained for 108 flowering individuals at two sites. The proportion of flowers maturing fruits was less than 10% at both sites. Inflorescences produced an average of 2,045±111 and 1,743±77 flowers at the two sites, initiated an average of 439±48 and 304±21 fruits, and matured an average of 194±16 and 162±10 fruits, respectively. Overall, 51% of the fruits which initiated development were aborted and abscized in an early stage of development.The hypothesis that fruit abortion serves to regulate seed yeild, matching fruit production to the available resources, yields three predictions which are borne out by our findings: (1) the proportion of initiated fruits which abort is an increasing function of the number of fruits initiated, (2) the number of fruits matured is an increasing function of estimates of the amount of resources available to support fruits, and (3) on any inflorescence, fruits initiated early have a lower probability of aborting than fruits initiated late.These results suggest that low fruit set in Yucca whipplei is caused by shortage of resources to support developing fruits, rather than by a shortage of pollinators. Possible explanations of the adaptive significance of regulating maternal investment via fruit abortion are mentioned, but further experimentation is required before any conclusions can be drawn.

Oviposition and pollination behavior of the yucca moth, Tegeticula maculata (Lepidoptera: Prodoxidae), and its relation to the reproductive biology of Yucca whipplei (Agavaceae).

The adult behavior of the yucca moth, Tegeticula maculata Riley, is finely tuned to the reproductive biology of its specific host plant, Yucca whipplei Torr. The female moths oviposit in the ovaries of the yucca flowers and actively pollinate the same flowers with pollen which they have collected previously. The selective pressures imposed on the moths by 1) the plant's need for pollen transfer via an insect pollinating agent, 2) its partial self-incompatibility, and 3) its ability to regulate seed set by aborting excess fruits, have molded the pollinator's behavior in such a way that its offspring have the greatest possible chance of surviving through the early larval stages. The evolutionary responses of the pollinator include the following: 1) the female moths consistently fly to a different plant after collecting pollen, thus insuring cross-fertilization of the flowers, 2) they always pollinate after depositing the first egg in a flower, but not necessarily after subsequent ovipositions, and 3) females emerging near the end of the flowering season frequently oviposit in developing seed pods, as opposed to open flowers which are more likely to be aborted by the plants.

Post-Fire Regeneration of Southern California Chaparral

Succession of chaparral shrubs was studied for the first 4 years after fire on the four major slope faces at three elevations in southern California. Although total cover fluctuated from year to year, shrub cover increased annually through the third year. There was little or no increase in shrub cover between the third and fourth years. Four years after fire the shrubs covered 55% ground surface at the highest elevation site but only 28% at the lowest elevation site. Shrub cover was similar between slope faces with one exception; at the lowest elevation site shrub cover was twice as great on the north and east-facing slopes as on the south and west-facing slopes. Post-fire recovery of shrubs was by seedlings and/or resprouts. Yucca whipplei was an exception in that it did not resprout once the aboveground parts were killed nor did seedlings establish after fire, however the aboveground parts of many Y. whipplei survived the fire. Species which reproduced entirely by seed did so in the first post-fire year from soil-stored seed with the exception of Ceanothus greggii at the highest elevation site. This species was entirely absent the first year after fire but abundant in the second year. Species producing both resprouts and seedlings varied from site to site in the proportion of resprouts:seedlings. Between 83–100% of the post-fire populations of Cercocarpus betuloides, Arctostaphylos glandulosa, and Xylococcus bicolor were resprouts whereas 12–13% of Ceanothus tomentosus were resprouts. For Adenostoma fasciculatum, resprouts constituted 27–54% of the population at the lowest elevation and 65–94% at the highest elevation; whereas, the Quercus dumosa population was 100% resprouts at the lowest elevation and 31–67% resprouts at the highest elevation. Data are presented on the height of resprouts and seedlings for all species at the end of the first post-fire year.

POST‐FIRE REGENERATION OF SOUTHERN CALIFORNIA CHAPARRAL

Succession of chaparral shrubs was studied for the first 4 years after fire on the four major slope faces at three elevations in southern California. Although total cover fluctuated from year to year, shrub cover increased annually through the third year. There was little or no increase in shrub cover between the third and fourth years. Four years after fire the shrubs covered 55% ground surface at the highest elevation site but only 28% at the lowest elevation site. Shrub cover was similar between slope faces with one exception; at the lowest elevation site shrub cover was twice as great on the north and east‐facing slopes as on the south and west‐facing slopes. Post‐fire recovery of shrubs was by seedlings and/or resprouts. Yucca whipplei was an exception in that it did not resprout once the aboveground parts were killed nor did seedlings establish after fire, however the aboveground parts of many Y. whipplei survived the fire. Species which reproduced entirely by seed did so in the first post‐fire year from soil‐stored seed with the exception of Ceanothus greggii at the highest elevation site. This species was entirely absent the first year after fire but abundant in the second year. Species producing both resprouts and seedlings varied from site to site in the proportion of resprouts:seedlings. Between 83–100% of the post‐fire populations of Cercocarpus betuloides, Arctostaphylos glandulosa, and Xylococcus bicolor were resprouts whereas 12–13% of Ceanothus tomentosus were resprouts. For Adenostoma fasciculatum, resprouts constituted 27–54% of the population at the lowest elevation and 65–94% at the highest elevation; whereas, the Quercus dumosa population was 100% resprouts at the lowest elevation and 31–67% resprouts at the highest elevation. Data are presented on the height of resprouts and seedlings for all species at the end of the first post‐fire year.

Determinants of fruit set in Yucca whipplei: Reproductive expenditure vs. pollinator availability.

The number of flowers produced by inflorescences of Yucca whipplei (Agavaceae) consistently exceeds the number of fruits produced by about one order of magnitude. To determine the factors responsible for low fruit set, the relation between pollinator availability, the amount of resources spent on reproduction (as indicated by inflorescence size), and the number of fruits matured was studied during 1978 and 1979 at 18 locations in chaparral, coastal sage scrub, and desert scrub communities of southern California.The following results support the conclusion that pollinators do not usually limit fruit production in Yucca whipplei. Rather, fruit production is limited by the amount of resources available to support developing fruits. (1) Fruit production is positively correlated with inflorescence size both within and between populations. The average size of inflorescence for a population is an excellent predictor of mean fruit production. Furthermore, 54% of the total variance in fruit production of individual plants can be explained by inflorescence size. (2) In contrast, although fruit production within most populations is positively correlated with an index of the number of pollinator visits to an inflorescence, the relative abundance of pollinators for a population is a poor predictor of mean fruit production, and only 9% of the total variance in fruit production can be explained by the visitation index. Furthermore, at four sites studied for two years, there was little change in average inflorescence size or fruit production from 1978 to 1979, despite large differences in relative abundance of pollinators at each of the sites. (3) Based on geographic proximity, and physiographic and vegetational similarities, study sites were grouped into regional clusters. Both inflorescence size and fruit production varied considerably between regions. Of the total variation in fruit production, 27% can be attributed to differences between regions. Most of this variation is the result of regional differences in inflorescence size, which in turn influence fruit production.Why does Yucca whipplei produce such large inflorescences if so few fruits can be supported? Two relevant hypotheses are discussed: (1) the floral display is the result of selection for pollen dissemination at the expense of fruit set; and (2) the floral display is the result of selection for a bet-hedging strategy either to increase the probability of adequate pollination when pollinators are unusually rare, or to allow individuals to support more fruits when resources are unusually abundant.

Biological Interrelationships of Moths and Yucca whipplei, by Jerry A. Mackie. University of California Press, Berkeley and Los Angeles. University of California Publications in Entomology, 1966, 42: 1-59; 7 figs.; 6 plates. Price 2.00 U.S. dollars.

Biological Interrelationships of Moths and Yucca whipplei, by Jerry A. Mackie. University of California Press, Berkeley and Los Angeles. University of California Publications in Entomology, 1966, 42: 1-59; 7 figs.; 6 plates. Price 2.00 U.S. dollars.

A Verification of the Occurrence of Yucca whipplei in Arizona

A Verification of the Occurrence of Yucca whipplei in Arizona