590 TEXTBOOK OF BOTANY 



It is evident that an aquatic environment includes most of the factors 

 of a land environment, but the intensity or quantity of any factor is 

 very different. Some raw materials essential to green plants, such as 

 light, carbon dioxide, and oxygen, are more often at a minimum in 

 water than in air. Rates of diffusion also are so much slower in water 

 that, in the absence of currents, the movement of dissolved gases to 

 plants may be so slow that both photosynthesis and respiration are 

 limited. On land transpiration often becomes destructive. Because of 

 these effects of the medium, some kinds of plants survive only in certain 

 places on land, others only in certain places in water. Plants that can 

 live both in water and on land are either microscopic plants or those 

 larger plants which have emergent leaves when growing in water. The 

 vertical spread of plants on the earth is diagrammatically represented in 

 Fig. 269. 



Under-water habitats are roughly of two kinds : fresh water, including 

 lakes and ponds, rivers and streams; and marine, such as oceans, seas, 

 gulfs, and bays. There are also inland bodies of water of high salt 

 content in the plains, semi-deserts, and deserts. 



The Fresh-water Environment 



The fresh-water environment includes all non-saline lakes and ponds, 

 rivers and smaller streams, swamps, marshes, and bogs. Each of these 

 types of habitat is considered separately. 



LAKES AND PONDS 



Lakes and ponds differ from each other chiefly in area, depth of water, 

 permanence, effects of winds and temperature changes, and depth of 

 light penetration. They represent the so-called standing water, in con- 

 trast to the flowing water of streams, although there may be surface 

 movements due to winds, changes in temperature, and deep springs. 



Light. The intensity of light at the surface of a lake is dependent pri- 

 marily on the latitude and the time of day. It may be modified locally 

 by clouds, fog, smoke, dust, and marginal shade. It is not, however, 

 the intensity of the light at the surface that is of importance in the 

 growth of submerged plants, but the light that actually penetrates the 

 water to the depth at which they live. 



The amount of direct sunlight that is reflected from a smooth water 

 surface ranges from 2 per cent when the sun is directly overhead to 

 nearly 100 per cent when the sun is near the horizon. Of the total 



