444 VEGETABLE PHYSIOLOGY 
normal ones. The most conspicuous instances of this are 
afforded by several species of the genus Viola. In one of 
these flowers the pollen grains often put out their pollen 
tubes while they are in the sporangia, and the tubes grow 
towards the stigma, penetrating it and reaching the ovules 
agin the case of the normal flower, fertilisation resulting 
in the same way. 
The process of pollination is followed in the ordinary 
course of events by the germination of the microspore or 
pollen grain. The facts that it grows upon the substratum 
of the stigmatic surface and that the resulting gametophyte 
or pollen tube is often of considerable length mark a great 
difference between it and the gametophytes of the vascular 
cryptogams. It becomes indeed a parasite feeding upon a 
host plant during the greater part of its development. 
The course of events in the germination of the pollen 
grain appears to be the following. At the outset it absorbs 
water from the moist surface of the stigma and swells, its 
protoplasm becoming generally more granular. It almost 
simultaneously absorbs such food material as the surface 
of the stigma can supply, usually some kind of sugar. 
Most pollen grains contain a certain amount of reserve 
food material, frequently starch or sugar, or both. The 
process of absorption is followed by the secretion of 
enzymes, which can act upon these reserve materials, the 
most prominent of which are diastase and invertase. The 
former seems to be the most widespread, but the latter is 
far from uncommon. In some cases both enzymes are 
developed. The outer coat of the grain then bursts, and 
the inner one begins to protrude, probably in consequence 
of the hydrostatic pressure set up by the water that has 
been absorbed. Usually only one such tube protrudes, 
though occasionally several are developed. Intra-cellular 
digestion of the reserve materials follows, and the tube 
grows at their expense. The increased nutrition ig fol- 
lowed by a further increase of the enzymes, which is 
sometimes preceded by a temporary diminution, This, 
