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 



