132 TEXTBOOK OF BOTANY 



the spores ; but in the pine the time at which the species 

 is distributed has been shifted from the period of the 

 spore to that of the seed. The seed is much better 

 adapted to this purpose than the spore, because it can 

 live for a long time without germinating, and because 

 it contains much food for the new plant. Note that a 

 seed is not a means of reproduction of the plant, as a 

 spore is. It is itself a little plant, wrapped up in tissues 

 belonging to the two previous generations. 

 The life cycle of the pine may be represented by a diagram 



(Fig. 77) not unlike those which have been used for the moss 



and the fern. 



157. Seed Plants : Gymnosperms. — Each member of 

 the most highly developed class of plants — the seed plants — 

 has a history much like that of the pine. The seed plants 

 as a group have adapted themselves so well to present con- 

 ditions upon the earth that they have spread over much the 

 greater part of its land surface. It is because they are so 

 well fitted to the conditions of existence that the largest and 

 longest-lived plants are seed plants. Seed plants are divided 

 into two distinct groups, named according to the ways in 

 which they bear their seeds. In the smaller and more primi- 

 tive group, to which the pine belongs, the seeds are produced 

 on the surface of a leaf (although a leaf of very special form) 

 or on a short branch. The members of this group are called 

 gymnosperms or naked-seeded plants. A much larger group 

 consists of those which, hke the cucumber, form their seeds 

 within a closed structure that becomes a friiit. Plants whose 

 seeds are shut up in this way are called angiosperms or hidden- 

 seeded plants. 



158. Different Kinds of Pines. — The pines include about 

 eighty species, which Hve almost entirely, in the north tem- 

 perate zone. Among them are some of the most valuable 

 timber trees. Of the species native to North America, the 

 best known is the white pine {Piniis strobus), forests of which 



