STRUCTURE, MORPHOLOGY, AND PHYSIOLOGY. 31 



roots. When these fall off from their axes they take 

 root in the ground. In some grasses (Poa stricta Lindb.) 

 we know only this apogamic condition ; others are very 

 seldom sexual (Deschamjpsia alpina K. & Sch., Festuca 

 Fuegiana Hook.), or the sexual stage is lacking in cer- 

 tain regions {Poa bulbosa L.). Poa alpina L. and Festuca 

 ovinia L. are alwa} r s sexual in lower countries, but in 

 high mountains in the north are frequently asexual. 

 Such leafy panicles have a very curled appearance. 



Geographical Distribution. — Grasses are found in all 

 parts of the globe, and belong to the outposts of phseno- 

 gamous vegetation in the polar regions, as well as to the 

 limits of perpetual snow of high mountains. The great- 

 est number of species is found in the tropical zone, but 

 the number of individuals is greater in the temperate 

 zones, where they are closely united to form extended 

 areas of turf that cover large meadows. The formation 

 of meadows is dependent upon a uniform rainfall or con- 

 tinuous irrigation. Grasses also predominate in steppes 

 and savannas, but here they grow in scattered sods or 

 tufts and do not entirely cover the ground. The savan- 

 na grasses are characterized by the fact that they 

 often reach above a man's head and are reed-like. The 

 Bambusece of the lowlands form an important part of 

 tropical forests, especially in regions of the Monsoon. 

 Upon South American mountains the bushy Bambusece 

 form an entirely closed cover. Aside from the weed- 

 grasses that have been generally distributed by com- 

 mercial intercourse and colonization, there are several 

 species that are cosmopolitan (for example, Heteropogon 

 contort lis R. & Sch., and Phragmites communis Trim), and 

 still others that are native to both hemispheres (also to 

 both tropics). Several species from the northern wooded 

 regions (Deschampsia flexuosa Trim, discolor R., atropur- 

 purea Scheele, Festuca ovina L., rubra L., elatior L., Poa 

 nemoralis L. and pratensis L.) are absent in the tropics, 

 and appear again unchanged in the antarctic regions ; 

 others appear in isolated places between on high moun- 

 tains of the tropics (PMeum alpinum L.) ; still others 

 (Alopecurus alpinus L., Trisetum subspicatum Beau v.) ap- 



