SEED-BEARING PLANTS 
429 
filling, the venter of the archegonium. .The pollen-tube 
passes between the neck-cells of the archegonium, but 
does not ordinarily enter the venter. The apex of the 
tube is ruptured, probably by internal osmotic pressure, 
and its entire contents are emptied into the cytoplasm 
of the egg. One of the sperm-nuclei unites with the 
egg-nucleus (June of the second season), and fertilization 
is accomplished (Fig. 319). 
p.r. 
FIG. 319. White pine (Pinus Strobus). Longitudinal section through 
an archegonium at the time of fertilization. Above the fusing nuclei are 
various other elements emptied into the egg from the pollen-tube. Col- 
lected June 21, 1898. X about 62. s.g, starch grains; p.r, prothallium; 
c.p.t, cytoplasm from pollen-tube; st.c, stalk-cell; t.n, tube-nucleus; s.n, 
sperm-nucleus; e.n, egg-nucleus; n.s, nutritive spheres. (After Margaret 
C. Ferguson.) 
382. Formation of the Seed. After fertilization the 
oosperm begins at once to develop, giving rise to three 
distinct structures; the pro-embryo, the suspensor, and 
the embryo-sporophyte. During the early divisions of 
the fertilized egg the male and female chromatins can be 
clearly distinguished (Fig. 320). At the same time the 
adjacent tissues of the oVule become transformed. A 
portion of the prothallus or gametophyte nourishes the 
developing embryo, but the large bulk of it becomes 
