516 A TEXTBOOK OF BOTANY [Cn. XIII 



all situations where plants can grow at all. Phylogenetically 

 they are without doubt descendants of the Pteridophytes, 

 as will be noted further below. 



The Spermatophytes fall into two Classes, sharply dis- 

 tinguished in several characters, - 



CLASS 1. GYMNOSPERMS : THE GYMNOSPERMS. 

 CLASS 2. ANGIOSPERM^E : THE ANGIOSPERMS. 



CLASS 1. GYMNOSPERMS : THE GYMNOSPERMS 



These are familiar in the Pines, Firs, Spruces, Cedars, 

 and other evergreen and cone-bearing trees, though the group 

 includes some with different characteristics. All have in 

 common, however, the feature that the seeds are naked, 

 that is, not inclosed by sporophylls (carpels) as in the 

 Angiosperms. Though of great evolutionary interest, the 

 class is not large, for it includes only some 500 living species. 

 Among them are included the most valuable of our timber 

 trees, and others which supply many minor needs of man. 

 A great many fossil species are known, and clearly the group 

 is declining in importance. 



The plant body exhibits the typical differentiation into 

 leaf, stem, and root, usually modified in adaptation to dry 

 situations in which mdst of these plants occur. Their 

 growth is exogenous, as in some ancient Pteridophytes. 

 The flowers are of the simplest character, lacking anything 

 like calyx or corolla, and are usually collected into unisexual 

 clusters most conspicuous in the cones. They are polli- 

 nated and disseminated almost wholly by wind. The 

 megaspores (embryo sacs) become completely filled with 

 the prothallus (endosperm), in which are buried the arche- 

 gonia. These are reduced to some short neck cells, and a 

 little-differentiated layer around the large egg cell. The 

 embryo develops on an elongated suspensor, and has usually 

 several cotyledons (polycotyledonous) . 



Ecologically the Gymnosperms are all terrestrial plants; 

 living in somewhat drier situations, upon the whole, than 



