REPRODUCTION 445 
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-sac 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 
undifferentiated 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, 
reaching 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 
detached from the parent plant before it has reached tho 
embryo-sac, from which it is separated by a bulky portion of 
