554 BOTANY OF THE LIVING PLANT 



on in a compact Strobilns, or Flower. Evidence of the steps of 

 segregation of the '' general purposes shoot " into distinct nutritive 

 and propagative regions may still be traced in favourable cases (see 

 p. 278). These regions once established took each its own independent 

 line of specialisation in the evolution of Seed-bearing Plants. The 

 vegetative region which appears first in the individual life commonly 

 develops normal foliage ; but under special conditions it may become 

 xerophytic, scandent, parasitic, or saprophytic, the adaptive nature 

 of the change being usually evident (Chapters XL, XII.). The propa- 

 gative region or Flower also became specialised in relation to its 

 functions. It appears later, and it is distal, since nutrition is necessary 

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

 removes the flower further 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 adaptability of the Seed-bearing Plants to subaerial 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 microspores of the Pteridophyte. 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 contents 

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

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

 formed and are passed on to their destination as the tube grows 

 (Chapter XVII.). 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 subaerial 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 



