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. 50 r. Tip of pollen tube 
, . .. 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 (), 
is made up of four tiers of cells, with four and abundant starch g rains <*> 
it ... n., . After COULTER. 
cells in each tier. The uppermost tier 
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 rrtay form a separate embryo. In any event, although 
several eggs may begin to form embryos, one embryo soon dominates 
and the others disappear. 
