456 SPERMATOPHYTES (SEED PLANTS) 
tube is eating its way through the nucellus to the female 
gametophyte. 
Male Gametophyte.— The male gametophyte forms within 
the pollen grain and its tube. At the time of pollination the male 
gametophyte commonly consists of four cells — two prothallial 
or vegetative cells, a generative cell, and a tube cell. At least one 
of the prothallial cells usually disintegrates and disappears early 
in the development of the gametophyte. This is the condition 
of the male gametophyte when the pollen is carried to the ovulate 
cone. Upon reaching the ovulate cones the pollen grains fall 
down to the base of the scales in the region of the ovules, and 
Fic. 405. — Seed structures of the Pine. A, a mature ovulate strobilus 
with scales spread apart to allow the seeds to escape. , a view of the inner 
side of a scale, showing the two seeds when mature. The wings of the seeds 
are a part of the scale and did not develop from the ovule. (, section through 
a pine seed, showing the female gametophyte (g), embryo (e), and seed coat (w). 
some lodge at the mouth of the micropyles, where they are caught 
in a drop of a mucilaginous secretion and drawn in close to the 
tip of the nucellus. In this position the pollen grains begin to 
develop tubes, which by means of an enzyme dissolve the nucel- 
lar tissue, using it as food and at the same time making a way for 
themselves. Cold weather finally checks the growth of the 
pollen tubes, and the male gametophytes now rest over winter. 
Early the next spring the pollen tube resumes its growth toward 
the archegonia, and the generative cell passes into the pollen tube 
and divides, forming two cells, one of which divides and forms the 
two sperms which now have the pollen tube as a passageway to 
the archegonia. The sperms reach the archegonia about the 
