SKIT. II 



PHANEROGAMIA 



483 



tissue only one, centrally placed macrospore-mother-cell is as a rule developed, all 

 the other cells remaining sterile. The mother-cell undergoes a tetrad-division (Fig. 

 443), and of the four resulting cells only one develops into an embryo-sac (macro- 

 spore). This, as it increases in size, first crushes its sister-cells and later the whole 

 sporogenous complex of cells. Meanwhile, by the repeated division of the nucleus 

 and protoplasm, the macrospore becomes filled with the tissue of the prothallium 

 (Fig. 444). The archegonia are formed at the apex of the prothallium ; each 

 consists of a large ovum and a short neck. As in the Pteridophytes a small ventral 

 canal-cell is cut off from the egg-cell shortly before fertilisation. Fertilisation takes 

 place by the entrance into the ovum of one of the generative cells, the development of 



A 



B 



,d 



l-'ii.. 446. Picea ewcbd (.-I-', F). Finns larlc'w (D, K, fH-1). A, mature ovum with its nucleus (nn) 

 and the ventral-canal-cell (</); fi, the male nucleus (x) within the ovum : ' , fusion of the 

 male and female nuclei ; D, the four nuclei produced by division of the nucleus of the embryo 

 have passed to the lower end of the ovum and are there in process of further division (only 

 two of the nuclei are visible in the section); K, four of the eight nuclei are contained in 

 independent cells, while the other four remain in the general cavity of the egg-cell ; F, further 

 division of the nuclei of the upper series ; (!, the cells of the lower tier have divided ; H, four 

 tiers of nuclei are present, those of the upper tier not being separated from the general cavity 

 of the egg-cell ; /, the elongation of the middle tier of four cells to form the suspensor (s) has 

 commenced. ' (A-<.' x 55, F x 110, after MIYAKE ; D, E, G-I x 104, after COULTER and 

 CHAMBERLAIN.) 



which has already been followed ; this enters from the pollen-tube, which penetrates 

 between the cells of the archegonial neck. The nucleus of the fertilised ovum 

 results from the fusion of the male and female sexual nuclei (Fig. 445) and the 

 protoplasm is in part derived from the male cell. The development of the embryo 

 from the fertilised ovum presents great differences in the several orders and even 

 genera, and the following description applies to the species of Pinus. 



By two successive divisions of the nucleus four nuclei are formed which pass to 

 tin- base of the egg-cell, where they arrange themselves in one plane and undergo 

 a further division. Cell walls are formed between the eight nuclei of this eight- 

 celled pro-embryo. The cells form two tiers, those of the upper tier being in open 

 communication with the cavity of the ovum. The four upper cells then undergo 

 another division, and this is followed by a similar division of the four lower cells. 

 The PRO-EMBRYO thus consists of four tiers each containing four cells, the cells of 



