174 STRUCTURE AND LIFE HISTORIES 
size. The cell- wall of the first division (in all of the family 
Polypodiaceae) is parallel to the axis of the archegonial 
neck. The second wall, at right angles to the first, di- 
vides the oosperm into four cells. The beginning of these 
divisions marks the beginning of the embryo. By further 
cell-divisions each of the first four cells develops a mass of 
embryonic tissue. The two cells on one side of the first 
wall formed represent, the one the embryonic stem, and 
the other the embryonic leaf, or cotyledon. One of the 
two cells on the opposite side of the first wall, develops 
into the embryonic root, while the other develops into an 
organ peculiar to the embryonic stage, and known as the 
foot (Fig. 131). The function of the foot is to absorb 
nourishment for the young embryo from the prothallus, 
by osmosis. The need of such an organ becomes ap- 
parent when it is recalled that the oosperm, and conse- 
quently the embryo, lie free in the venter of the arche- 
gonium, without any organic or structural connection 
with the prothallus. This necessary connection is early 
established by the foot. 
159. Growth of the Embryo. As the embryo continues 
to grow, the root develops first. The advantage of this 
will become evident when we remember that the primary 
and most fundamental need of the young plant is water, 
which is taken in by the roots. The next most funda- 
mental need is nourishment, and as plant food is manufac- 
tured in chlorophyll-bearing organs, and usually in 
leaves, we would expect the early development of leaves. 
Such is the case, the growth of the first leaf being second- 
ary only to that of the root, and in advance of the stem. 
The development of the stem follows, and finally spore- 
bearing leaves appear (Fig. 132). We then have an 
