114 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, i.e., 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. 

 P- 10). 



Respiration 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 (cf. 

 p. n). 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 1 (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. 



