376 THE SPERMATOPHYTES 



SUBDIVISION II. THE ANGIOSPEEMS, OK 

 ANGIOSPERMS 



357. The angiosperms. The angiosperms (meaning seeds in 

 a vessel) are distinguished by the fact that the ovules are devel- 

 oped in a closed case (ovule case or ovary) formed by .the car- 

 pels, sometimes alone but often together with adjacent tissue of 

 the stem. This immense assemblage of plants, with more than 

 120,000 species, forms the greater part of the earth's vegetation 

 and includes the most successful groups, dominating most of 

 the land floras. It is a much more varied assemblage than the 

 gymnosperms, and successful in every vegetation form (herb, 

 shrub, or tree). The angiosperms adapt themselves to all sorts 

 of life conditions, some of them being aquatics, others covering 

 the meadows, prairies, and heaths, certain groups entering the 

 deserts, and the trees forming forests generally accompanied 

 by undergrowths of shrubs. They occupy the highest points of 

 plant evolution, but along a great many very divergent lines, for 

 some of the culminating groups are the grasses, the hardwood 

 trees, the composite groups, the orchids, etc. 



The general structure of the angiosperms, including the roots, 

 stems, leaves, flowers, and fruits, together with many principles 

 of plant physiology best illustrated in this group, have been 

 described in Part I. This account will consider chiefly the life 

 history, with especial reference to the gametophyte generations 

 and significance of the flower. 



358. The angiosperm flower. The essential structures of 

 the angiosperm flower (Fig. 301), as of the gymnosperms, are 

 the stamens (microsporophylls) and the carpels (megasporo- 

 phylls) ; but in addition to these some accessory parts are gen- 

 erally present, which are either modified leaves of the plant, 

 or sometimes stamens and carpels that have become sterile. 

 These accessory parts constitute the perianth (Fig. 301, p), 

 situated on the stem just below the stamens and carpels, and 

 are generally showy structures, but also protective, at leact in 



