J Integr Plant Biol. ›› 1957, Vol. 6 ›› Issue (1): -.

• Research Articles •    

A Cytological Study on the Endosperm Development of Wheat

Yang Miao-hsien   

Abstract: The present report embodies a part of the investigation of the endosperm of wheat. It was conducted at the Department of Biology, University of Peking. -. The material used was Triticum vulgate var. erythrospermum. Collections were made in the springs of 1955 and 1956. Paraffin sections were cut serially. For the early stages endosperms were also dissected out and either mounted in toto, or split open, spread out and then mounted. Smear prepardations were also made. In addition, material freshly dissected out were examined in the living condition under a phase contrast microscope. In the present paper material is presented in 3 sections: 1. Formation and development of the endosperm, 2. Multiplication of the endosperm nuclei by mitosis and non-mitotic methods’ and 3. Behavior of the antipodal tissue and nuclear migration. 1. Formation and development of the endosperm. The endosperm of wheat is of the nuclear type. The primary endosperm nucleus divides mitotically (Pl. Ⅱ, fig. 4). A free nuclear stage follows. At this time the endosperm is a protoplasmic bag containing a large central vacuole. It is in the shape of a pear in external appearance. At the micropylar end the endosperm protrudes into the nucellar tissue to form a sort of haustorium. In this the proembryo resides (Pl. Ⅰ, fig 2). Nuclei of the endosperm are ellipsoid or spherical in shape, measuring 40–45 μ in the large diameter. A few are many times larger or smaller (Pl. Ⅰ, figs. 3–4). It contains one to 10 nucleoli or even more. Nucleoli multiply by fission or budding. in the fixed material, there appears a hyaline shell around each nucleolus which is not stainable. In the living material, however, no such shell exists. The living nucleolus is larger than in fixed material. It is, therefore, believed that the “shell” is an artifact produced by differential shrinkage of the nucleolus and the surrounding nucleoplasm. In the vicinity of the antipodal tissue fusion between free nuclei were observed (PI. Ⅰ, Figs. 5–6). Cell wall formation of the endosperm is initiated about 2 days after fertilization. It starts around the proembryo and progresses toward the chalazal end. Endosperm nuclei are more numerous at the region of the antipodal tissue but here the cell formation tags. Phragmoplast and subsequently cell plate and cell wall appear between 2 young sister nuclei resulting from a mitosis, but they also arise elsewhere between 2 nuclei which are not sisters (Pl. Ⅰ, figs. 7–8). A noteworthy feature in young cells of the endosperm is that neighboring cells are connected by thick plasmic strands (Pl. Ⅱ, fig. 2). 2. Multiplication of the endosperm nuclei. In the early stage of endosperm development, nuclei multiply solely by means of mitoses. Frequently all free nuclei of an endosperm were undergoing division at the time of fixation. Mitoses follow an orderly sequence. When nuclei in the vicinity of the antipodal tissue are in the metaphase, those farther away are in anaphase and those still farther removed are in telophase (Pl. Ⅱ, fig. 3). It appears that the antipodal tissue exerts some influence over cell division. It may be due to some enzyme secreted by that tissue. Among the dividing nuclei, polyploid ones were observed. They posses 2–5 times as many chromosomes as the normal triploid nuclei (Pl. Ⅲ figs. 3–4; PI. V fig. 33). These giant nuclei are also in the neighborhood of the antipodal tissue and probably had their origin in nuclear fusions. In the later stage of endosperm development, mitotic divisions presist, but now the are sporadic occurrences. About 2 days after fertilization cases of non-mitotic multiplication of nuclei appear in addition to the mitotic method. Non-mitotic divisions are not frequent but their occurrence is regular and they are found in every specimen at this stage of development. They occur most often in the region lying between the proembryo and the antipodal tissue. Non-mitotic divsions assume various forms: (1) by transverse constriction of the nucleus, (Pl. Ⅳ, figs. 5–6), (2) by longitudinal split (Pl. Ⅳ, figs. 7–11), (3) by budding (Pl. Ⅳ. figs. 11–16) and (4) by quartering. In this last mentioned manner 4 arms arc sent out from a nucleus and from this cross-shaped figure, 4 daughter nuclei result (Pl. Ⅴ, figs. I–2). There is also a problematical form of nuclear multiplication. from an amoeboid nucleus, bits are possibly shed, which grow into new nuclei. Observed facts tend to show that mitotic and non-mitotic methods of nuclear multiplication are closely associated and may be interconvertible. Nuclei are generally in the prophase condition, when they start to divide non-mitotically by constriction (Pl. Ⅳ, figs. 3–4). Cases were also observed when 2 newly formed daughter nuclei plainly arisen from mitotic division divided again this time non-mitotically (Pl.Ⅳ, figs. 1–2). 3. Behavior of the antipodal tissue and migration of nuclei. The antipodal tissue occupies a lateral position in the embryo sac (Pi. Ⅴ, fig. 7). After fertilization antipodal cells increase in size and number. They multiply both by mitotic and non-mitotic division (Pl. Ⅴ, figs. 5–6). Nucleoli in the nucleus also multipy. 5 or 6 days after fertilization, when starch grains begin to be formed in the endosperm cells, the antipodal tissue starts to disorganize. Their nuclei lose their normal structure, nucleoli disappear and the disintegrated nuclear matter forms irregular lumps or long strips. Next, cellwalls break down. Nucleoplasm streams into the endosperm tissue through intercellular spaces (PI. Ⅵ, figs. 1–2). Some penetrates the wall and enters the endosperm ceil. Evidently the disorganized antipodal tissue furnishes nutritive material for the endosperm, at the time when the latter is actively building starch. At the time when the antipodal tissue becomes disorganized, nuclei of some endosperm cells also migrate into adjacent ones (Pl. Ⅵ, figs. 4–6). Migration of nuclear material occurs most frequently in the neighborhood of tile disorganized antipodal tissue. Observations of the various cytological events that take place in the course of the endosperm growth and development leads to the view that these events are associated with the physiology of the development of the endosperm.

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