78 WEATHERWAX: GAMETOGENESIS IN ZEA MAys 
The aim in presenting here the meager data available is to show 
the great rapidity with which the pollen tube grows. It is inter- 
esting to note that Jensen (13, p. 15) finds a somewhat greater 
time (32 to 40 hours) between pollination and fecundation in 
wheat, although the style of the latter is much shorter than in 
maize. 
It is not to be inferred that there is at any one time a living 
pollen tube reaching from the stigma hair to the embryo sac; and 
this statement is probably very generally true of all angiosperms. 
Observation of the pollen tube in the silk is difficult, but it is 
probable that there is not more than a centimeter or two of it 
alive at any one time; and the living portion seems to become 
shorter and broader as it approaches the ovule. It is in most 
cases practically impossible to find any trace of the pollen tube in 
_ the tissue through which it has passed. 
One of the difficulties in attempting to observe fecundation 
is in being able to identify the sperms when they reach the embryo 
sac. The protoplasm of the pollen tube, as it grows down through 
the silk, is so dense and so coarsely granular that it is almost im- 
possible to distinguish with certainty the sperms and the tube 
nucleus. Both can be made out with a fair degree of certainty, 
however, in the less dense contents of the tube in the ovary cavity 
as it approaches the embryo sac; and they are, of course, visible in 
the pollen grain before germination. The tube nucleus is irregular — 
in size and shape and does not seem to have a definite membrane. 
There is no visible difference between the two sperms. They 
are very small, slender crescents, with the ends drawn out to very 
fine points (Fics. 15, 16). The nucleus, which seems to consist 
almost wholly of chromatin, constitutes the greater part of the 
cell and forms the middle of the crescent; no nucleolus has been 
observed. There is only a small quantity of cytoplasm, and noth- 
ing of its details can be made out. 
The size of the sperm is remarkably correlated with the long 
distance through which it must be carried from the pollen grain 
to the embryo sac. Practically all varieties of maize are fertile 
inter se; and this means that any pollen grain, even though it be 
produced by a plant characterized by short silks, is capable of 
producing a pollen tube as long as is necessary, as is determined 
by the length of the silk of the variety pollenized. 
