PHYSIOLOGICAL BASIS OF ACHENE DORMANCY IN POLYGONUM CONVOLVULUS (POLYGONACEAE)

Germination of dormant achenes of wild buckwheat (Polygonum convolvulus L.) was promoted at 25 C if they were stratified at low temperatures. Preincubation at either 2 C or 10 C promoted subsequent germination at 25 C equally, although the period of time to reach maximum germination was shorter for the 2 C treatment. Moreover, a preincubation treatment of a daily alternating cycle 2 C for 20 hr and 10 C for 4 hr promoted germination at 25 C more than either temperature alone. Removing portions of the hard pericarp and testa did not promote germination of dormant achenes at 25 C except when the portion of those structures covering the tip of the radicle was removed. This suggests that the structures covering the embryo do not prevent germination by restricting the movement of water or gases but rather restrict growth mechanically. Complete removal of the pericarp promoted germination, but much higher germination was obtained when the testa was also removed, indicating that the pericarp may not be the main factor in dormancy. Thus the role of low temperature in the loss of dormancy in wild buckwheat achenes may be to promote the production of hydrolytic enzymes that lower the mechanical resistance of pericarp and testa and/or increase the embryo growth potential beyond some threshold level and thereby enable the radicle to overcome the resistance to growth imposed by the structures covering the embryo.     

LEAF DEVELOPMENT OF RUMEX PATIENTIA L. (POLYGONACEAE) EXPOSED TO UV IRRADIATION (280-320 NM)

Two factors which affect leaf ontogeny and ultimate leaf size: (1) the rate and duration of cell expansion, and (2) the rate and duration of cell division, were examined for their role in the slower early leaf growth rate and the smaller size of fully expanded leaves of plants exposed to ultraviolet-it (UV-B 280-320 nm) radiation. Rumex patientia L. was grown in controlled environment chambers under enhanced UV-B radiation (equivalent to daily solar UV-B irradiation at 40°N latitude in mid-May with an atmospheric ozone concentration of 0.20 atm-cm) and control treatments. The pattern of growth as expressed in changes of mean cell size in two distinct cell types, tissue cell density, and length of the entire leaf blades are consistent with the hypothesis that the radiation primarily affects cell division rather than cell expansion. Furthermore, it appears that the radiation probably alters the rate rather than the duration of the cell division phase. An understanding of the mechanism of radiation damage should facilitate prediction of how this stress may interact with other stresses to which plants are normally subjected. Species with normally prolonged periods of cell division during leaf expansion may be particularly impacted if solar UV radiation were intensified as a result of atmospheric ozone reduction.     

新疆木蓼属新种

Atraphaxis jrtyschensis C. Y. Yang et Y. L. Han, sp.nov.     

A BIOSYSTEMATIC STUDY OF THE POLYGONUM HYDROPIPEROIDES (POLYGONACEAE) COMPLEX

The Polygonum hydropiperoides complex includes P. hydropiperoides Michx., P. opelousanum Riddell ex Small, P. setaceum Baldwin ex Ell., and P. hisutum Walter. Studies of morphology, cytology, and crossing compatibility were conducted with specimens from North Carolina, South Carolina, Georgia, and Florida. A multivariate cluster analysis using 212 specimens and 34 characters indicated that P. hirsutum and P. setaceum are each morphologically distinct while P. hydropiperoides and P. opelousanum are morphologically indistinct from one another. The chromosome counts of 2n = 20 for P. setaceum and P. hirsutum, and 2n = 40 for P. hydropiperoides and P. opelousanum are the first reported for the respective species. Experimental hybridizations produced the fertile hybrids P. hydropiperoides x opelousanum and P. setaceum × hirsutum. Results of these studies and population structure, habitat, and pollination studies suggest that P. hydropiperoides, P. setaceum, and P. hirsutum are each distinct species while P. opelousanum is indistinct and should be merged with P. hydropiperoides.     

FLORAL DEVELOPMENT IN CAESALPINIA (LEGUMINOSAE)

Utilizing scanning electron microscopy, we studied the early floral ontogeny of three species of Caesalpinia (Leguminosae: Caesalpinioideae): C. cassioides, C. pulcherrima, and C. vesicaria. Interspecific differences among the three are minor at early and middle stages of floral development. Members of the calyx, corolla, first stamen whorl, and second stamen whorl appear in acropetal order, except that the carpel is present before appearance of the last three inner stamens. Sepals are formed in generally unidirectional succession, beginning with one on the abaxial side next to the subtending bracts, followed by the two lateral sepals and adaxial sepal, then lastly the other adaxial sepal. In one flower of C. vesicaria, sepals were helically initiated. In the calyx, the first-initiated sepal maintains a size advantage over the other four sepals and eventually becomes cucullate, enveloping the remaining parts of the flower. The cucullate abaxial sepal is found in the majority of species of the genus Caesalpinia. Petals, outer stamens, and inner stamens are formed unidirectionally in each whorl from the abaxial to the adaxial sides of the flower. Abaxial stamens are present before the last petals are visible as mounds on the adaxial side, so that the floral apex is engaged in initiation of different categories of floral organs at the same time.     

FLORAL DEVELOPMENT IN MYRISTICA (MYRISTICACEAE)

Myristica fragrans and M. malabarica are dioecious. Both staminate and pistillate plants produce axillary flowering structures. Each pistillate flower is solitary, borne terminally on a short, second-order shoot that bears a pair of ephemeral bracts. Each staminate inflorescence similarly produces a terminal flower and, usually, a third-order, racemose axis in the axil of each pair of bracts. Each flower on these indeterminate axes is in the axil of a bract. On the abaxial side immediately below the perianth, each flower has a bracteole, which is produced by the floral apex. Three tepal primordia are initiated on the margins of the floral apex in an acyclic pattern. Subsequent intercalary growth produces a perianth tube. Alternate with the tepals, three anther primordia arise on the margins of a broadened floral apex in an acyclic or helical pattern. Usually two more anther primordia arise adjacent to each of the first three primordia, producing a total of nine primordia. At this stage the floral apex begins to lose its meristematic appearance, but the residuum persists. Intercalary growth below the floral apex produces a columnar receptacle. The anther primordia remain adnate to the receptacle and grow longitudinally as the receptacle elongates. Each primordium develops into an anther with two pairs of septate, elongate microsporangia. In pistillate flowers, a carpel primordium encircles the floral apex eventually producing an ascidiate carpel with a cleft on the oblique apex and upper adaxial wall. The floral ontogeny supports the morphological interpretation of myristicaceous flowers as trimerous with either four-sporangiate anthers or monocarpellate pistils.     

MORPHOLOGICAL AND GENETIC VARIATION IN ECOLOGICALLY CENTRAL AND MARGINAL POPULATIONS OF RUMEX ACETOSELLA L. (POLYGONACEAE)

Six ecologically central (old field) and marginal (strip mine) populations of the hexaploid Rumex acetosella were collected, grown under uniform conditions, and examined for genetic and morphological variation. Extensive electrophoretic variation was found in alcohol dehydrogenase and phosphoglucose isomerase, while other enzymes surveyed showed little or no variation. Hedrick's genotypic measure of identity revealed mean values of 0.506 for central populations, 0.836 for marginal populations, and 0.633 for comparisons of central with marginal populations. Alcohol dehydrogenase phenotypes had significantly fewer electromorphs per individual in marginal populations. Clones of individuals from both environments were subjected to different watering regimes. No significant differences in root/shoot ratio, leaf number, total leaf area, or relative growth rate were found between strip mine and old-field individuals within each watering treatment, although significant differences were found between watering treatments. There are small, but significant amounts of isozyme differentiation between central and marginal populations, while there was no such differentation for morphological characters.     

POLLEN DEVELOPMENT IN OENOTHERA BIENNIS (ONAGRACEAE)

Development of the exine and viscin threads in Oenothera was studied by a combination of transmission electron microscopy and field emission scanning electron microscopy. Exine formation is initiated in the early tetrad stage by plate-like structures of preexine formed evenly around the microspore within a callosic wall. In the late tetrad stage, an endexine accumulates on white-lined lamellae underneath the preexine. After dissolution of the callosic wall, the preexine develops into beaded ektexine which is differentiated into an undulate tectum-like layer and columellae-like components. Interwoven fibrous strings connect the developing ektexine and the surface of the tapetal cells, and later develop into the viscin threads. These developmental processes imply that the columellae-like components are different in structure from the columellae of other angiosperms and that the formation of viscin threads is associated with the tapetal cells.     

LEAF DEVELOPMENT IN SENECIO ROWLEYANUS (COMPOSITAE)

Prolonged apical growth of the leaf primordium and the presence of distinct marginal meristems do not occur in Senecio rowleyanus. Intercalary cell divisions accompanied by radial expansion of derivatives from an adaxial meristem account for the spherical shape of the leaf. The “window” in the lamina marks the position of the adaxial meristem and precludes interpretation of the leaf as being unifacial. Stomata are mesoperigenous and anomocytic in type. Schizogenous secretory canals occur in both the leaf and the stem, and their association with vascular bundles is discussed. The anatomy of the leaf is interpreted in terms of xeromorphy.     

FLORAL DEVELOPMENT IN GYMNOTHECA CHINENSIS (SAURURACEAE)

The development of the inflorescence and flowers are described for Gymnotheca chinensis Decaisne (Saururaceae), which is native only to southeast China. The inflorescence is a short terminal spike of about 50–70 flowers, each subtended by a small bract. There are no showy involucral bracts. The bracts are initiated before the flowers, in acropetal order. Flowers tend to be initiated in whorls of three which alternate with the previous whorl members. No perianth is present. The flower contains six stamens, and four carpels fused in an inferior ovary containing 40–60 ovules on four parietal placentae. Floral symmetry is dorsiventral from inception and throughout organ initiation. Floral organs are initiated in the following order: 1) median adaxial stamen, 2) a pair of lateral common primordia which bifurcate radially to produce two stamen primordia each, 3) median abaxial stamen, 4) a pair of lateral carpel primordia, 5) median adaxial carpel, 6) median abaxial carpel. This order of initiation differs from that of any other Saururaceae previously investigated. The inferior ovary results from intercalary growth below the level of stamen attachment; the style elongates by intercalary growth, and the four stigmas remain free. The floral structure of Gymnotheca is relatively advanced compared to Saururus, but its assemblage of specializations differs from that of either Anemopsis or Houttuynia, the other derived genera in the Saururaceae.     

LEAF DEVELOPMENT IN DOXANTHA UNGUIS-CATI (BIGNONIACEAE)

Leaf structure in Doxantha unguis-cati is polymorphic. The usual mature compound leaf is composed of two lanceolate leaflets and a terminal tripartite spine-tendril. Leaf primordia are initiated simultaneously in pairs on opposite flanks of the shoot apical meristem by periclinal cell divisions in the third subsurface layer of the peripheral flank meristem. Two leaflet primordia are the first lateral appendages of the compound leaf. Initiation of these leaflet primordia occurs on the adaxial side of a compound leaf primordium 63–70 μm long. Lamina formation is initiated at the base of a leaflet primordium 70–90 μm long and continues acropetally. Mesophyll differentiation occurs in later stages of development of leaflets. The second pair of lateral appendages of the leaf primordium differentiate as prongs of the tendril. Initiation of the second pair of lateral appendages occurs on the adaxial side of a primordium approximately 168 μm long. Acropetal procambialization and vacuolation of cells extend to the apex of tendrils about 112 μm long, restricting the tendril meristem to the adaxial side of the primordium and resulting in curvature of the tendril. The tendril meristem is gradually limited to a more basipetal position as elongation of apical cells continues. Initiatory divisions and early ontogenetic stages of leaflets and tendrils are similar. Their ontogeny differs when the lateral primordia are approximately 70 μm long. Marginal and submarginal initials differentiate within leaflets but not in tendrils. Apical growth of tendrils ceases very early in ontogeny as compared with leaflets.     

PERIDIAL DEVELOPMENT IN GLOMUS MOSSEAE (GLOMACEAE)

Unispored sporocarps of a Kansas isolate of G. mosseae at all stages of development were prepared for light and scanning electron microscopy to reveal the process involved in peridium formation and spore release. Five stages were identified: 1) peridial hyphal branch origination from below the sporophore attachment; 2) continued peridial hyphal branching and weft formation; 3) adherence of peridial hyphae to the spore wall and completion of peridial expansion into mature state; 4) separation of the primary (evanescent) spore wall layer from the spore and the inner peridial layer; and 5) release of the de-peridiate spore. Spores in stage 3 were surrounded by a two-layered peridium consisting of thinner-walled, parallel-arranged hyphae next to the spore and thicker-walled, jigsaw puzzle-arranged hyphae to the outside; the latter of which collapsed during stages 4 and 5. Fine radial canals and lignituberlike ingrowths were seen in both the de-peridiate spore wall and, for the first time, in the shed peridium. Comparison of these results with studies of related sporocarpic genera lead to the conclusion that the process of peridium formation in G. mosseae is similar to that of G. coronatum, but that other sporocarpic Glomus representatives must be characterized before valid comparisons may be drawn.     

CENTRUM DEVELOPMENT IN DIDYMOSPHAERIA SADASIVANII (PLEOSPORALES)

Development of a typical pseudoparaphysate centrum in Didymosphaeria sadasivanii Ramachandra-Reddy indicates that this ascomycete is properly placed in the Pleosporaceae despite the fact that forcible discharge of ascospores from bitunicate asci has not been demonstrated. The relatively thin-walled asci releasing ascospores within the ascocarp in D. sadasivanii, as in Cochliobolus spp., probably were derived by reduction from the bitunicate type. Ascocarps matured on malt agar slants but developed more rapidly and normally on autoclaved alfalfa stems inoculated in medicine bottles and transferred to moist filter paper in large petri dishes when covered by mycelium.     

CONE AND OVULE DEVELOPMENT IN SCIADOPITYS (TAXODIACEAE-CONIFERALES)

Ontogeny of seed cones of Sciadopitys, with special reference to the ovule-supporting structure, is studied in material collected in Japan and Massachusetts. Cones are initiated as lateral or terminal structures in summer and complete the formation of most organs before winter. Bract development is well advanced before ovule-supporting structures are initiated. Continued cone development involves the formation of a narrow ridge of tissue in the axil of each fertile bract. This ridge develops a series of nine (but up to 12) apical lobes in centrifugal order, each of which is the primordium of a future tooth on the ovuliferous scale. Ovules are initiated as outgrowths of the adaxial surface of each lobe so that there is a one-to-one ratio between lobes and ovules. Intercalary extension of the ovuliferous scale itself (distally) and the common base of the bract and ovuliferous scale (proximally) greatly extends the complex. The ovuliferous scale eventually exceeds the subtending bract and its apex becomes recurved. Bracts each have a single trace, but each ovuliferous scale has a pair of traces that proliferate distally to irrigate ovule and scale lobe. Intercalary growth results in recurvature of the ovule trace. The organization of the cone is directly comparable with certain Permian fossils. Sciadopitys also seems unique within the Taxodiaceae in its centrifugal development of the ovule-supporting complex.     

PATTERNS OF GENICULAR DEVELOPMENT IN AMPHIROA (CORALLINACEAE)

Two types of genicular development (designated Types I and II) are present in South African species of Amphiroa. Genicula of both types originate when medullary tissue near branch endings softens by decalcification. In Type I decalcification does not progress to the branch surface and the joint therefore does not become flexible until the cortex, which remains calcified, cracks and partly sloughs. This type occurs in all species of Amphiroa in South Africa except one, and possibly occurs in those in the rest of the world as well. In Type II decalcification continues centrifugally to the surface of the branch; this results in a geniculum comprising medullary and cortical tissue. As far as is known this type occurs only in Amphiroa ephedraea. Type I appears to be more primitive than Type II.     

FLOWER DEVELOPMENT IN DIOECIOUS SPINACIA OLERACEA (CHENOPODIACEAE)

Spinacia oleracea (Chenopodiaceae) is a potential model system for studies of mechanisms of sex expression and environmental influences on gender in dioecious species. Development of the male and female flowers and inflorescences of spinach were studied to determine when the two sex types can be distinguished. We found that female inflorescence apices are significantly larger than those of the male. Flower primordia are similar in size prior to perianth initiation, but the male primordia develop at a faster rate. Another distinguishing feature at this early stage is the larger bract subtending the female primordium. The two flower types become readily distinguishable when the perianth initiates. Male flowers produce four sepals and four stamens in a spiral pattern in close succession. Female flowers produce two alternate perianth parts that enlarge somewhat before the gynoecium becomes visible. There are no traces of gynoecia in male flowers or of stamens in female flowers. We propose that plant sex type is determined before inflorescence development, prior to or at evocation.     

ENDOSPERM DEVELOPMENT IN THE DATE PALM (PHOENIX DACTYLIFERA) (ARECACEAE)

Date seeds were sampled at regular intervals from pollination (March) to mature fruit (September) and processed for light microscopy and SDS-PAGE. Seed fresh weight rose until early June and then declined slightly through September due to a continuous decrease in water content. Cell wall formation started in May in the free nuclear endosperm and proceeded centripetally from the inner integument to the seed center. Wall thickening in each cell started in cell corners and showed a layered appearance with calcofluor white staining. It started in early June in the center of the seed and proceeded centrifugally such that the outer cells showed cell wall thickening in late June. Thickened cell walls were soft and PAS positive at inception, but staining disappeared and hardness increased during wall maturation. Cell elongation in the radial direction accompanied wall thickening. Protein body formation started after cell wall thickening and followed the same centrifugal developmental pattern. Mature protein bodies occurred in even the outermost cells by early July. No further structural changes occurred after this time. The high molecular weight storage proteins appeared in late June, which is when protein bodies had formed in all but the outer endosperm cells; however, these proteins did not appear simultaneously and minor changes in protein bands continued until maturation. α-Galactosidase activity was present in the developing endosperm and peaked at 13 wk after pollination. The data suggest that the thickened wall is deposited as a highly substituted galactomannan, but that most of the galactose side branches are clipped off presumably by α-galactosidase during cell wall polymerization.