ch. in] Distribution 23 



CHAPTER III 



DISTRIBUTION OP BRITISH GRASSES 



A study of the distribution of grasses affords both interest 

 and profit. While several of our common species are vagabonds, 

 i.e. generally distributed without any apparent preference for 

 one habitat more than another, it is nevertheless true that most 

 grasses do show a decided preference for certain definite conditions 

 as regards soil, moisture, degree of shade, etc. Indeed as we have 

 already seen in the previous chapter many species possess obvious 

 adaptations for growing in dry and exposed situations (Xero- 

 phytes). 



Other species (Hydrophytes) are adapted for aquatic con- 

 ditions, e.g. Glyceria aquatica, Digraphis, etc. The grasses of 

 this class have large expanded leaves, generally with stomata on 

 both surfaces, and large air cavities are often present in the 

 sheaths, etc. 



A few grasses are Halophytes, i.e. adapted for soils rich in 

 saline constituents, e.g. Psamma, Elymus, etc. In these trans- 

 piration is checked by the thickening of the cuticle of the leaves, 

 by the inrolling of the blades, and by the production of a waxy 

 substance over the entire surface of the plants. 



But a knowledge of this subject is of practical value to those 

 interested in agriculture. To the student at first it may simply 

 afford assistance in finding or identifying the species, but after 

 some experience he will be able to reverse the process, and to 

 form a useful opinion about the physical properties and agri- 

 cultural value of land from observation of the grasses thriving 

 upon it. The presence of much sand, clay, chalk, or humus in a 

 soil, or again the natural tendency of a field to be wet or dry, are 

 all indicated with a very fair amount of accuracy by the healthy 

 growth of certain species, or better still by groups of species. 

 Broadly speaking our grasses may be grouped according to (a) the 

 relative amount of moisture, and (b) the relative amount of shade 

 which they prefer. For further information on this subject see 

 Bibliography, Nos. 3, 20, 22 and 32. 



