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A TEXTBOOK OF THEORETICAL BOTANY 



arises, which is composed of basidia and paraphyses arranged side by side. 

 The basidia are club-shaped cells which are at first binucleate, but as they 

 mature the nuclei fuse. It will be seen that the two nuclei which first come 

 together at the union of the ( + ) and ( - ) mycelia finally unite in the basidium. 

 The paraphyses are similar in shape to the basidia, but there is no nuclear 

 fusion in them. 



After fusion the nucleus in the basidium undergoes a meiosis, resulting 

 in the formation of four monoploid nuclei (Fig. 317). At the end of the 



* fe«^ ,„^^ -isilmt. 



C D 



Fig. 317. — Psalliota catupestris. Cytology of the basidium. A, Formation of sterigmata 

 and migration of nuclei into them. Nuclear divisions can be seen in some basidia. 

 B and C, Development of young basidiospores. D, Mature basidiospores. A second 

 division appears to be in progress in the basidium which may give rise to a second crop 

 of spores. 



second division centrosomes * attach themselves to the wall at points which 

 become the positions of origin of the sterigmata. These four sterigmata 

 are narrow tubes from the ends of which the basidiospores develop, and 

 into each of which one monoploid nucleus enters. This migration of the 

 nucleus is apparently efl^ected by the initial migration of the centrosome, 

 which leaves behind a stainable thread, possibly analogous to an astral ray, 

 which draws the nucleus through the narrow sterigma into the spore. Mean- 

 while the basidium becomes vacuolate and the pressure produced by this 

 may not only cause the flow of the nucleus and cytoplasm into the spore, 

 but may also assist in the final discharge of the latter. 



* Centrosomes are minute granules commonly associated with the nucleus in animal 

 cells and found also in some of the lower plants. They form the points of origin of the spindle 

 fibres during mitosis. 



