WEATHERWAX: GAMETOGENESIS IN ZEA Mays fas 
THE POLLEN TUBE 
As has already been noted (24, p. 487), the generative cell of 
the pollen grain has divided, forming two very small, crescent- 
shaped sperms, before the dehiscence of the anther. The pollen 
grain finally comes to rest on the silk where it is probably held by 
some sticky exudation that may be instrumental in initiating 
germination. The pollen tube emerges very soon and makes its 
way down through one of the stigma hairs to the body of the silk 
(Fic. 14). Here it pushes its way between the parenchyma cells, 
absorbing food from them, no doubt, until it reaches one of the 
vascular bundles, which it follows toward the ovary. 
At the time of its emergence from the pollen grain, the tube 
is greatly exposed to desiccation, and this continues until it has 
made its way into the body of the silk. This, together with the 
fact that the pollen grains themselves dry up very readily and are 
seldom viable for more than a day or two, probably accounts for 
the importance to the corn crop of warm rains at the time of polli- 
nation. 
In some kinds of corn the silks may attain a length of fifty 
centimeters or more, and there are probably few species of plants 
in which the pollen tube has farther to grow. Consequently, it 
has been an interesting problem to determine the time elapsing - 
between pollination and fecundation. A number of experiments 
were made to determine this, and the best results were obtained 
with a small variety of sweet corn with silks about twenty-five 
centimeters in length. Silks that had been properly protected 
were pollenized at 2:45 P.M. on a warm, foggy day in early Sep- 
tember, with pollen that had been shed on the forenoon of the 
same day, and ovules and segments of silks were fixed at various 
intervals for two or three days after pollination. It was found 
that within two hours the pollen tubes were well established in the 
silks. This result has also been verified by several experiments in 
the laboratory (Fic. 14). Ovules fixed twenty-five hours after 
pollination showed fecundation and stages just before (Fic. 16) 
and just after fecundation. These results could, of course, be 
modified by extensive experiments with silks of different lengths 
and pollen of different ages and at different temperatures, degrees 
of humidity, etc., but that is rather the problem of the physiologist. 
