454 VEGETABLE PHYSIOLOGY 
it, either exclusively, or in cases in which cross-pollination 
fails to be effected. Only one of these need here be alluded 
to: this is cleistogamy, or the production of special flowers 
which do not open, in addition to the 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 as in 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 as the surface of 
the stigma can supply, usually some kind of sugar. Most 
pollen-grains contain a certain amount of reserve food, 
frequently starch or sugar, or both. The process of absorp- 
tion 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 pro- 
trude, 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 
