76 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



seen from such obvious examples as the tall stems of forest plants 

 which reach for light, or the long roots of desert plants which seek 

 out water. 



Before considering these adaptive modifications, we must deal 

 with those physiological attributes which are most significant in 

 aff^ecting plant distribution, for they are fundamental to plant 

 geography. 



Water is essential for the life and growth of plants, being a con- 

 stituent of their bodies and necessary for many of their life-processes. 

 Consequently its availability is among the most important factors 

 of a plant's environment. Where there is no water, plants cannot 

 long persist in an active state ; although seeds may survive bone- 

 dry for many years, they need water to germinate as the plants do 

 to grow. Between a desertic lack and an aquatic superabundance 

 of water, which are extremes that only suitably adapted plants can 

 withstand, there are various degrees of water availability to which 

 particular species are accustomed and often limited. The need for 

 water largely determines the distribution of plants on the face of 

 the earth, as we may see when passing from any lastingly dry area 

 to a wet one, when the flora and vegetation will change drastically. 



The actual eff^ect of available water may be complicated by con- 

 ditions, such as temperature and atmospheric humidity, that affect 

 its utilization within the plant — for example through controlling 

 absorption by the roots, movement in the stem, or loss from the 

 leaves, etc. Particularly susceptible to atmospheric changes are the 

 microscopic pores (stomata) through which most water-vapour and 

 other gaseous exchange takes place between the internal tissues of 

 higher plants and the atmosphere at large. Consequently the all- 

 important water economy of the plant is affected by conditions in 

 the surrounding air as well as by the availability of water in the 

 soil. 



Temperature is another of the most important factors of the 

 plant's environment. Particular plants require particular tempera- 

 ture-ranges for their life-processes and normal development, and, 

 different temperatures being characteristic of different climates, such 

 requirements widely limit the geographical distribution of plants. 

 And along with plants, of course, go the vegetation-types which 

 they make up. 



Under otherwise constant conditions each plant has an optimum 

 temperature at which it does best, and, on either side of this, a 

 range extending to maximum and minimum temperatures beyond 



