GRASSES HITCHCOCK 299 



mass or plume. In crabgrasses {Diffifaria), Bermuda grass 

 (Cynodon), and goosegrass {Eleusine) the spikelets are borne in 

 slender one-sided digitate (fingerlike) spikes. 



Most grasses have perfect flowers (stamens and pistils in the same 

 flower). Those with unisexual flowers (containing only stamens or 

 only pistils) may be monoecious (the two kinds of flowers on differ- 

 ent parts of the same plant, as in corn) or dioecious (the two kinds 

 of flowers on different plants, as in buffalo grass {Buchloe) and 

 saltgrass {Distichlis) ) . 



POLLINATION 



Basically, the pollination and fertilization of grasses is the same 

 as in other kinds of flowering plants. The pollen produced by the 

 stamens is transferred to the stigmas of the pistil, the process being 

 called pollination. There the pollen germinates and (by means of 

 a minute tube) grows down through the style into the ovary and 

 finally to the germ cell of the ovule. The protoplasm of the end of 

 the pollen tube fuses with the protoplasm of the germ cell, fertilizing 

 it. The fertilized ovule develops into a seed. (See p. 298.) 



Showy or fragrant flowers are chiefly pollinated by insects, which, 

 seeking nectar, become covered with pollen in one flower and carry 

 it to the next one visited, where some of it is dusted on the stigmas. 



Darwin and others have shown that cross-fertilization as a rule 

 produces more vigorous offspring than does self-fertilization. The 

 means by which cross-j^ollination is effected in plants is of great 

 interest, and many botanists have given much time to its study. 



Grasses may be cross-pollinated or self-jDollinated. The latter 

 process is more common in grasses than in most other plants. (See 

 p. 300.) 



For cross-pollination the grasses depend largely on the wind, 

 which, however, is so wasteful of pollen that, as in all wind-pol- 

 linated plants, the amount produced is greatly in excess of the 

 amount used. The wind must carry clouds of pollen in order that 

 a few grains may reach the stigmas (only one grain being needed 

 for the fertilization of an ovule). This over-production of pollen 

 of grasses and some other wind-pollinated plants, especially ragweed, 

 afflicts man with the so-called " hay fever ". 



There are among the grasses certain habits or structures which 

 favor cross-pollination. In dioecious grasses self-pollination cannot 

 occur. In monoecious grasses cross-pollination is the rule, though 

 self-pollination is possible. The corn plant is a familiar example of 

 the effect of the separation of the sexes on the same individual 

 (monoecism). Under cultivation corn is grown in large areas. The 

 wind carries the pollen from the tassels of one group of plants to 

 the silk of plants lying to the leeward of these but allows little tO' 



