SPERMATOPHYTES 



225 



m 



and Cupressineae the male cells are equal, since both may function by 

 the tube entering the chamber of an archegonium complex. The tip of 

 the pollen tube, just before fertilization, contains the two male cells and 

 also (usually in advance of them) the stalk and tube nuclei (fig. 501). 



Fertilization. The general features of fertilization are as described 

 for the Taxaceae, the pollen tube acting as a carrier of the male cells 

 to the archegonia, in addition to its old 

 function as an haustorium. 



Embryo. In the development of the 

 embryo (figs. 502-509), free nuclear divi- 

 sion occurs until four to sixteen nuclei are 

 formed, and sooner or later become placed 

 at the bottom of the egg. With the next 

 nuclear division walls appear, and division 

 of cells continues until three or four tiers 

 of cells are formed, the tiers containing 

 approximately the same number of cells. 

 This proembryo, therefore, by no means 

 fills the cavity of the egg, as in the pre- 

 ceding groups of gymnosperms, the greater 

 bulk of the egg being a large reservoir of 

 surplus food material. 



The proembryo of Pinus may be used 

 to illustrate the general structure of the FIG. 501. -Tip of pollen tube 



r . ... ^ of Pinus, just before fertiliza- 



proembryo and the functions of its different t ion, containing the two male cells 

 regions (figs. 502-509). This proembryo (m), stalk and tube nuclei (w), 



is made up of four tiers of cells, with four and abundant starch S rains <*> 



-,.,._,, . After COULTER. 



cells in each tier. The uppermost t i er 



consists of four cells, open (without walls) towards the food reservoir of 

 the egg. The next tier, which is the part of the proembryo that 

 remains within the egg, constitutes the so-called rosette. The third 

 tier of four cells forms the suspensor, each cell elongating enormously, 

 so that the four-celled suspensor becomes a long and tortuous filament. 

 At the tip of the suspensor, thrust by its elongation deep into the 

 endosperm, is the lowest tier of four cells, which forms the embryo 

 (figs. 509, 510). All four of these cells may form one embryo, or each 

 of the. four cells may form a separate embryo. In any event, although 

 several eggs may begin to form embryos, one embryo soon dominates 

 and the others disappear. 



