568 



BOTANY 



PART II 



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 (G\ and this is followed by a similar division of the 

 four lower cells (H). The PRO-EMBRYO thus consists of four tiers, each 

 containing four cells, the cells of the upper tier being continuous with 



K 



FIG. 547. Picea excelsa (A-C). Pinus laricio (Z)-A~). A, Mature ovum with its nucleus (cm) and the 

 ventral-canal-cell (d). B, The male nucleus (sri) within the ovum. C, Fusion of the male 

 and female nuclei. D-K, Description in text ; s, suspensor. (A-C x 55, after MIYAKE ; 

 D-H x 200, after KILDAHL ; 7, A" x 104, after COULTER and CHAMBERLAIN.) 



the remaining portion of the ovum. In the further development of 

 the three lower tiers the middle tier elongates to form the SUSPENSOR 

 (K, s), pushing the terminal tier from which the embryo will arise into 

 the tissue of the prothallium or endosperm ; the cells of the latter are 

 filled with nutritive reserve material. 



In other genera a separation of the four rows of cells takes place, 

 and each bears a young embryo. As a rule, however, only a single 

 embryo continues its development in each macrospore, although several 

 archegonia may have been fertilised. The embryo is formed of the 



