CHAPTER XV 



THE SEED PLANTS: FORMS AND LIFE 

 HISTORIES 



TAKEN as a whole the Seed Plants represent the most 

 complete adaptation to terrestrial life that we find 

 within the plant kingdom ; and, correspondingly, 

 they are the plants which now dominate the face of 

 the earth. On the vegetative side this adaptation is 

 represented by the broad features of organ and tissue 

 development described on pp. 241-2, which are common 

 to all the vascular plants, but are carried to a higher 

 pitch of specialisation in the Seed Plants than in 

 the Pteridophyta. On the reproductive side it is 

 represented primarily by the seed itself and by the 

 structures and processes leading up to and accompanying 

 its development. The seed is a direct metamorphosis 

 of the ovule : in other words the ovule changes directly 

 into the seed. And the ovule is simply a megaspor- 

 angium containing a single megaspore, which has 

 germinated, not by producing any structure external 

 to itself, but by producing the female gamete or egg 

 inside the megaspore as a result of the division of 

 the megaspore nucleus. The male gamete, produced, as 

 in the heterosporous Pteridophytes, inside the micro- 

 spore, is brought to the neighbourhood of the mega- 

 sporangium still contained in the microspore, and then, 

 by the growth of a germ tube put out from the microspore, 

 to the megaspore and to the egg itself. The process 



