GEASSES HITCHCOCK 305 



quickly appear in openings in timber due to fire, to landslides, or to 

 other agencies that destroy the trees previously occupying the space. 

 Banks of rivers or sandbars exposed by receding waters are soon occu- 

 pied by annuals. Sandy soil in regions of moderate rainfall also fur- 

 nish a satisfactory substratum for them. When the sand is wet, the 

 seeds germinate quickly, and the plants are often able to produce their 

 own crop of seed before the soil dries out. In desert regions annuals 

 spring up quickly after a rain and mature seed before the eifects of 

 the rain have passed. " Six- weeks " grasses of several annual species 

 are abundant after rains in the arid regions of our southwestern 

 States. 



PERENNIALS MORE PERSISTENT THAN ANNUALS 



Perennials come to maturity more slowly than do annuals and 

 usually do not produce seed the first year. But once established they 

 can hold the ground against annuals. The difficulty of keeping crab- 

 grasses out of lawns composed of perennial grasses may seem to con- 

 tradict this statement. But a lawn, except in a region of plentiful 

 fogs and drizzle, is a highly artificial creation. The frequent mowing 

 weakens the plants, for it is the foliage that makes the food, and the 

 constant watering causes the roots to spread close to the surface in- 

 stead of deep into the soil. When crabgrass and yellow bristlegrass 

 take possession of a lawn it is because the perennials fail to withstand 

 the hard conditions and leave unoccupied spots. The countless seeds 

 of annuals are ever present to take advantage of any opening. 



GRASSES A DOMINANT FAMILY 



Grasses are one of the dominant families of plants. In number 

 of genera and species they are exceeded by the sunflower family 

 (Compositae), the orchids, the legumes, and the madder family (Ru- 

 biaceae), but in the number of individuals they probably exceed all 

 other families. They are distributed throughout the world, from 

 pole to pole, and from sea level to alpine summits, wherever there is 

 a substratum (free from snow for a part of the year) on which they 

 can grow. Only in tropical rain forests are they scarce. Here are 

 found several species with broad thin blades that are able to grow 

 in the deep shade. But even in the rain forests narrow-leaved grasses 

 grow in the occasional openings where the light is more abundant. 

 In the American tropical rain forests there are several kinds of 

 climbing bamboos forming beautiful lacy curtains along trails and 

 streams. 



Grasses vary in size from an inch to 100 feet or more (giant bam- 

 boos). They may be erect, creeping, or climbing. The climbing 

 species have no special organs, such as tendrils, nor do they twine. 



