100 



PLANTS AND MAN 



tive and nourishing organ, the seed. The evolutionary significance 

 of the seed habit is clear when we discover that over half (about 

 160,000) of all the plant species can be grouped into this phylum, 

 and that land vegetation is made up almost exclusively by them. 

 The Spermatophyta or Seed Plants are subdivided into two 

 classes. One group, or class, of less than 1000 species, includes the 



Fig. 70. — Cycads are semi-tropical low-growing Gymnosperms with fern-like 

 leaves and cone-like reproductive structures. 



more primitive members in which the seeds are produced on 

 exposed cone scales. These are the Gymnosperms, in which no 

 fruit surrounds the seed, and which are therefore called the naked- 

 seed plants. The other group, made up of the remaining species 

 of the phylum, is that of the Angiosperms, in which the seed lies 

 within a protective fruit (see p. 76). 



