CLASSIFICATION OF ANGIOSPERMS. 



467 



the seed being always firmly united with the thin pericarp (except 

 in Sporobolus, Eleusine, etc.). The embryo is situated at the 

 base, on the outer convex surface of the seed, outside the endo- 

 sperm. On germination the cotyledons remain in the seed. 



The endosperm contains numerous starch grains and oil, while 

 the gluten layer around the endosperm contains proteins. The 

 number of layers of gluten- or aleurone-containing cells varies in 

 the different cereals. In corn, wheat, and rye it consists of but 

 a single layer ; in oat (Fig. 247) and rice, of i or 2 layers ; while in 

 barley it is made up of 2 to 4 layers. 



The Grasses comprise about 3500 species and are distributed 

 in all parts of the world. While most of the plants are grass-like, 



FIG. 254. Diagrams of cross-sections of monocotyledonous flowers: t, stem of plant; 

 f, bract; s, sepals or outer circle of perianth; p, petals or inner circle of perianth; a, stamens; 

 c, ovary. A, regular flower of the lily; B, irregular flower of iris. C, flower of an orchid, 

 in which 1 is the position of the lip and & S of the two staminodes. After Warming. 



still some of them, as the bamboos of the Tropics, become quite 

 tall, having woody siliceous stems and' bearing many branches in 

 the axils of the leaves. The grasses yield the cereal grains forming 

 so large a proportion of the food of man, and forage constituting 

 the food of many of the lower animals. The following are some of 

 the important cereals: Wheat (Triticum sativum and its varie- 

 ties), corn (Zea Mays), oat (Avena sativa), rice (Oryza saliva), 

 barley (Hordeum sativum and its varieties), rye (Secale cereale). 

 A number of the species yield a sweet cell-sap from which cane 

 sugar is made, of which the most important are the sugar cane 

 (Saccharum officinarum) and sorghum (Andropogon arundina- 

 ceus saccharatus and other varieties). (Consult pp. 148, 156, 198.) 



