梅雌蕊发育和受精作用的研究

以梅细叶青品种为试验材料,在南京地区,雌蕊心皮原基分化高峰期在１０月上旬,１２月下旬,形成胚珠原基。次年２月下旬至３月初,胚囊分化渐趋势成熟,蓼型胚囊。子房内着生２个半倒生胚珠,通常仅１个发育为种子。胚珠具厚珠心,形成珠孔塞和珠心冠。３月上旬开花,传粉后２－３天,花粉管经引导组织进入子房,５天左右,多数胚囊中已完成双受精,８天,胚乳游离核开始出现,１３天左右的材料,原胚开始形成,胚胎发育属紫菀型,     

DEVELOPMENT OF CHAMBERED PITH IN STEMS OF PHYTOLACCA AMERICANA L. (PHYTOLACCACEAE)

An integrated microscopic (light and electron microscopy) and macroscopic investigation of chambered pith development was made of Phytolacca americana L. Terminal internodes have a solid pith cylinder in contrast to the alternating diaphragms and chambers occurring in subjacent pith. Macroscopically, chambers and diaphragms of any one internode are of equal size. Microscopically, diaphragms vary in height within an internode (from 1–6 cells high). Nevertheless, all diaphragms become thicker circumferentially (5–12 cells high) and connect with long files of intact peripheral pith cells. Diaphragm cells have a large centrally positioned vacuole with a thin, parietal layer of cytoplasm; nuclei, mitochondria, endoplasmic reticulum, and unidentified organelles differentiate in the cytoplasm of diaphragm cells. Although schizogenous activity has most often been implicated as the mechanism by which chambered pith develops in vegetative organs of angiosperms, the results of this study show that cavities in pokeweed result from both schizogenous and lysigenous mechanisms. Schizogeny is suggested by the fact that central pith cells of terminal internodes are longer and thinner walled than peripheral pith cells arranged in vertical files, thus indicating elongation of cells as a possible result of internode elongation. The precise developmental pattern and arrangement of chambers and diaphragms also suggest schizogenous processes. Lysigenous or enzymatic activity is indicated by the fact that cavities are bounded by broken cells, and wall fragments and organelles are often found within enlarging cavities. Chamber formation occurs continuously acropetally and centrifugally in the central pith. A comparison of diaphragms is made with Liriodendron tulipifera and Juglans nigra in an attempt to resolve differences in structure and terminology regarding the differentiation of chambered and diaphragmed pith.     

CHANGES IN THE ULTRASTRUCTURE OF XYLEM PARENCHYMA CELLS OF PEACH (PRUNUS PERSICA) AND RED OAK (QUERCUS RUBRA) IN RESPONSE TO A FREEZING STRESS

An ultrastructural investigation was conducted of xylem parenchyma cells of peach (Prunus persica [L.] Batsch.) cv. Harbrite and red oak (Quercus rubra L.) in response to a freezing stress. Freezing curves of xylem tissues, as determined by differential thermal analysis, were used to predict temperatures at which both living and dead cells would be observed. Tissues were exposed to low temperatures (-15 to -35 C) and fixed in a frozen state at -10C and at thawing. Current models of the freezing behavior of supercooled plant cells suggest that xylem parenchyma cells behave as individual water droplets. This implies that cells are unresponsive to the presence of low temperature and extracellular ice until internal nucleation triggers lethal, intracellular freezing. For these reasons, deep supercooling has been described as an avoidance mechanism. Results of this study confirmed earlier reports that xylem parenchyma cells freeze as individuals or in small groups. Individual cells, however, did not exhibit a neutral response. Instead, a range of responses was observed that included internal and external vesiculation, deep invaginations of the plasma membrane, and the formation of electron-dense deposits external to the plasmalemma. In general, our observations suggested that the cells responded to a dehydrative stress. Results are discussed in context of the biophysical data associated with deep supercooling phenomena and compared to responses of cells that exhibit extracellular freezing.     

DEVELOPMENT OF NORMAL AND SEEDLESS ACHENES IN CICHORIUM INTYBUS (COMPOSITAE)

Cross- and partially cross-pollinated capitula of Cichorium intybus (Compositae, Lactuceae) were examined for a study of normal and seedless fruit development respectively. Embryos develop according to the Asterad pattern, and the free-nuclear endosperm becomes cellular 15–17 hrs after pollination. A zone of disorganized cellular material surrounds the embryo sac at anthesis, and, in normal achenes, this zone expands as the seed develops. Initially the developing seed elongates and comes into contact with the top of the ovary by 48 hrs. In contrast to this pattern, the ovule in developing seedless achenes degenerates within 72 hrs. Irregularities, such as an abnormally proliferating endothelium, embryo formation without endosperm, and endosperm formation without an embryo often accompany this degeneration. Differentiation of the pericarp in seeded achenes begins between 48 and 72 hrs, starting at the apex and proceeding basipetally; in seedless fruits the process is similar though initiated somewhat later. The normal pericarp at maturity exhibits a pigmented exocarp, a broad mesocarp of thick-walled lignified cells, and a tenuous endocarp. In seedless achenes the fruit coat is similar except that the exocarp is colorless and the cells of the mesocarp are relatively small.     

STRUCTURE AND DEVELOPMENT OF THE CHLOROPLAST IN CHLAMYDOMONAS : I. THE NORMAL GREEN CELL

The cytoplasmic organization of a normal green strain of the alga Chlamydomonas reinhardi has been investigated with the electron microscope using thin sections of OsO₄ fixed material. The detailed organization of the chloroplast has been of special interest. The chloroplast, a cup-shaped organelle, surrounded by a double membrane, consists of: (1) discs about 1 micron in diameter, considered to represent the basic structural unit of the chloroplast, and each composed of a pair of membranes joined at their ends to form a flat closed vesicle; the discs are grouped into stacks resembling the grana of higher plants; (2) matrix material of low density in which the discs are embedded; (3) starch grains; (4) the pyrenoid, a non-lamellar region associated with starch synthesis, and containing tubules which connect with the lamellae; (5) the eyespot, a differentiated region containing two or three plates of hexagonally packed, carotenoid-containing granules, located between discs, and associated with phototaxis. In addition to the chloroplast, the cytoplasm contains various membranous and granular components, including mitochondria, endoplasmic reticulum, and dictyosomes, identified on the basis of morphological comparability with structures seen in animal cells. The nucleus, not investigated in detail in this study, contains a large, granular nucleolus and is surrounded by a nuclear envelope which is provided with pores and exhibits instances of continuity with the endoplasmic reticulum of the cytoplasm.     

POLAR TRANSPORT OF GROWTH-REGULATORS IN PITH AND VASCULAR TISSUES OF COLEUS STEMS

Movement of IAA-C¹⁴ and 2,4-D-C¹⁴ through cylinders of known size and histology was compared using liquid scintillation counting. Both auxins showed strongly polar movement, even through pith parenchyma cut from Coleus internode #5, the youngest internode to have ceased elongation. The polar movement was correlated with sizable elongation of the excised cylinders. Velocities of basipetal movement for a given auxin, as determined by the intercept method, showed small or negligible differences between pith and “corner” cylinders. (Corner cylinders comprised mostly vascular tissue, plus some cortical, pith, and epidermal cells.) For IAA, basipetal velocities ranged from 2.1 to 3.3 mm per hr; for 2,4-D, they were 0.6–0.8. For both auxins there was much more net loss into corner than into pith cylinders, a difference associated with the fact that corner cylinders showed 10 times as many cells in transection. More 2,4-D moved basipetally through corner than through pith cylinders and the reverse was true of IAA. By chromatographic evidence, all the radioactivity in the basal receiving blocks was still associated with the auxin molecules.     

THE ROLE OF LIGHT IN HISTOGENESIS AND DIFFERENTIATION IN THE SHOOT OF PISUM SATIVUM. III. THE INTERNODE

Thomson, Betty F., and Pauline Monz Miller. (Connecticut Coll., New London.) The role of light in histogenesis and differentiation in the shoot of Pisum sativum. III. The internode. Amer. Jour. Bot. 50(3): 219–227. Illus. 1963.—Seedlings of Pisum sativum were grown under constant conditions and exposed daily to red or white fluorescent light or kept in total darkness. Counts and measurements of internodal cells in both transverse and longitudinal directions show that light does not alter the sequence or pattern of tissue differentiation, including the sequence of xylem maturation within the vascular bundle. Light does accelerate the rate of a constant course of differentiation. Light advances the time of division and enlargement of cortex, xylem, phloem, and pith cells in the longitudinal direction but reduces both the final number and the final length attained in all cases. It is concluded that light accelerates all phases of shoot growth and differentiation and that cell division and elongation in the later phases of internodal growth are reduced by light because of accelerated cell maturation.