REPRODUCTION 455 
nutrition is followed by a further increase of the enzymes, 
which is sometimes preceded by a temporary diminution. 
This, however, does not last long, and soon a considerable 
increase can be observed. In some pollen-tubes such as those 
of the Lily, in whose pollen starch granules are abundant, 
the process of the digestion of the starch can be observed 
taking place as the granules move along the tube during 
its elongation. Soon an excretion of the enzyme into the 
tissues of the style takes place, and the reserve materials 
which are stored in the style are gradually digested as the 
tube advances, thus ministering to its nutrition, absorption 
of the products of the digestion being effected by the tube. 
The latter in most cases makes its way to the micropyle of 
the ovule, and by this channel reaches the embryo-sae or 
megaspore. At this period the latter contains its gameto- 
phyte, or prothallium, at the apex of which the oosphere 
or female gamete occurs. The tip of the pollen-tube comes 
in contact with the wall of the embryo-sac close to the 
oosphere. It then contains two gametes, which are un- 
differentiated masses of protoplasm, each with a very large 
nucleus. The separating walls become deliquescent and. 
are absorbed, and one of the male gametes fuses with the. 
oosphere, forming as before a zygote, while the other often. 
fuses with the definitive nucleus, as has already been. 
described. 
In a few cases the pollen-tube makes its way to the base 
of the embryo-sac and burrows through its contents, reach- 
ing the oosphere from below. This has been observed 
particularly in Casuarina and in certain of the forest trees. 
A few variations of this process have been observed 
among the Gymnosperms. As the plants of this group are 
all diclinous, self-pollination is of course impossible. The 
agent of pollination is usually the wind, and the pollen- 
grain in these plants falls upon the micropyle of the ovule, 
there being no ovary and consequently no stigma. The 
growth of the tube is slow, sometimes extending over 
several months. Indeed in some cases the sporangium is 
