IIl4 THE FLOWERING PLANT. 
it away. (d.) Polypetalous flowers are the most likely to be 
blown open and damaged by wind. The sweet-pea illustrates 
very well how such flowers are protected. In the first place, the 
peduncles are extremely strong and at the same time flexible, 
so that they yield without injury to gusts of wind. Again, the 
petals are firmly locked together at their bases by means of 
knobs and corresponding hollows. In this special case of the 
papilionaceous flower, the most important part is specially 
strengthened, 7.c., the two lowest petals are united into the keel 
(p. 88). Lastly, the standard serves as a sail, causing the flower 
always to point away from the wind. Further examples are 
unnecessary, aS many devices are sufficiently obvious on a little 
consideration. 
Flowers are not organs of nutrition, but any chlorophyll they 
may contain assists in the building up of organic matter (cf, 
256), 
: See ion is carried on very vigorously by flowers, and some 
crowded inflorescences, such as those of arum, can conveniently 
be used for demonstrating this process. Flowers also commonly 
excrete, or pass out to the exterior, other substances besides 
carbon dioxide formed by the breaking down of protoplasm (¢/. 
p. 11). Such, for example, are nectar and the volatile substances 
to which the odour of many flowers is due. ‘These excretions are, 
in a sense, ‘‘ waste products,’”’ but they are not useless. The same 
remark applies to blastocolla, the digestive juices of “ insecti- 
vorous plants,” and the viscid substances on stems, leaves, &c., 
by which insects are kept off. 
We now come to the main function of the flower, that of true 
reproduction. This differs from vegetative reproduction (p. 43) 
in that special reproductive cells or spores are formed. The 
pollen grains, when ripe, are liberated by dehiscence of the anther, 
and then, by various means, are transferred, In gymnosperms to 
the micropyle of the ovule, in angiosperms to the stigma, This 
transference of pollen is called pollination. The viscid substance 
excreted by the stigma stimulates the pollen grains to a sort of 
growth ; that is to say, each of them sends out a delicate pollen 
tube! (fig. 49), which grows down through the style by forcing 
its way between the delicate cells of the conducting tissue, or by 
traversing the slime in the canal, if this is present. Arrived at 
the ovary, the tube makes its way to the micropyle of an ovule 
and applies its tip to the apex of the nucellus. The two co- 
operating cells now absorb some of the protoplasm from the 
1 Pollen grains laid in a drop of weak (not more than 10 per cent.) sugar 
solution on a microscopic slide, and placed in the dark for a few hours, will 
be found to have emitted pollen tubes. 
