MESSMATES, MUTUALISTS, AND PARASITES 85 



number of small bodies known as Ovules, destined to become 

 seeds, and the most important part of an ovule is a minute egg- 

 cell or ovum. On the top of the pistil is a rough and sticky 

 surface, the Stigma. 



Pollination is a necessary preliminary to fertilization, and 

 consists of the transfer of ripe pollen-grains to the stigma. 

 Supposing this to have been effected, a thread-like pollen-tube 

 grows from each grain into the cavity of the carpel until it reaches 

 that part of the ovule where the egg-cell is located. A nucleus- 

 containing fragment of protoplasm (equivalent to a sperm) from 

 the tip of the tube is then transferred to this cell, with which it 

 fuses. The egg-cell, thus fertilized, develops into an embryo, and 

 the rest of the ovule undergoes certain modifications, the total 

 product being a seed. 



A stigma may be pollinated by grains developed in the stamens 

 of its own flower (self-pollination), or by grains derived from other 

 flowers (cross-pollination). The latter, since it is followed by 

 cross-fertilization, is the more desirable event, and the actual 

 transfer of pollen from flower to flower is effected by water, 

 wind, or animals, according to the nature of the arrangements 

 which have been evolved. We are here only concerned with 

 animal-pollinated (zoophilous) flowers, and in the large majority 

 of these the agents are insects. Many of the characters of insect- 

 pollinated (entomophilous) flowers have been evolved with re- 

 ference to the attraction and reception of suitable guests. It was 

 at one time the general belief that the varied odours and hues of 

 flowers came into existence simply and solely for the delectation 

 of mankind ; as a matter of fact their significance is utilitarian, and 

 has reference to the needs of plants themselves. For scent and 

 colour are the means employed to attract insects (and sometimes 

 other animals) capable of doing the work of cross -pollination. 

 It .is fortunate that the odours generally commend themselves 

 to us, but this is not always so, for certain flowers (e.g. some 

 Arums) smell like carrion, the object being to attract those flies 

 which revel in putridity. As to colour there is of course a great 

 variety, and some tints appeal to special visitors. Typical " bee 

 flowers ", for example, are commonly reddish-purple, purple, or 

 blue. Other forms depend upon dusk-loving insects, especially 

 moths, for their cross-pollination, and these are white or pale in 

 hue, which gives them the best chance of being seen (fig. 1079). 



