490 BOTANY OF THE LIVING PLANT 



its form to the circumstances (Chapters X., XL). The propagative 

 region or Flower also became specialised in relation to its functions. 

 It appears later, and is distal, since nutrition is necessary before 

 propagation can be carried out. This distal position, while it is 

 further removed from the water-supply, offers the best opportunity 

 for transfer of pollen, whether by wind, or by animal agency. The 

 functions of the Flower are, to produce sporangia ; by its structure 

 to offer facilities for pollination, with fertilisation as its consequence ; 

 to protect and nurse the new germs up to the period of ripeness of 

 the seed ; and, finally, to secure seed-dispersal. The means by which 

 these ends are attained are almost infinitely various. Examples are 

 described at length in Appendix A. It is the high degree of adapta- 

 bility of the Seed-bearing Plants to sub-aerial conditions, so as to 

 secure these ends, that has given them their supremacy. The pollen- 

 grains, usually dry and dusty, retain essentially the character of the 

 micro-spores of the Pteridophyta. They thus allow of either self- 

 pollination, or of inter-crossing in various degrees, in organisms 

 themselves non-motile. Commonly they are exposed at the time 

 of flowering to dry air and full sunlight. The antheridial mother-cell 

 within each grain is protected by the cuticularised and often coloured 

 coat of the grain from injury by drought or intense light : its products 

 are not set free into water as in the Amphibious Pteridophytes, but 

 pass into the security of the pollen-tube : there the male gametes are 

 formed, which are passed on to their destination as the tube grows 

 (Chapter XVI.). Similarly, the ovum is never exposed. Its pro- 

 tection against all risks is secured by a succession of tissue-envelopes. 

 The carpel, one or two integuments, and finally the nucellus all take 

 their part in this duty. The ovum itself, a primordial cell not differ- 

 ing essentially from the exposed egg of Fucus, thus deeply sunk in 

 living tissue, is immune to the risks of sub-aerial life. It is in a 

 position, when fertilised, to draw its supplies during the nursing 

 period from the embryo-sac, and the surrounding envelopes. Such 

 conditions, combined with the effective and often elaborate means of 

 distribution of the ripe seeds already described (Chapter XVIIt.), 

 account for the Seed-Plants becoming the chief factor in Land- 

 Vegetation. The disabihties of the gametophyte for land-life have 

 been evaded. The more adaptive sporophyte has developed specially 

 so as to protect the most vulnerable points in the cycle of life, viz. 

 the period of fertilisation, and the first stages of development of the 

 embryo. Thus the sporophyte has become virtually the Plant of the Land, 

 and the gametophyte a mere vestige. 



