274 Science of Plant Life 



of the. first plants that grew on the earth, and the shallow 

 water in which the fresh-water algae live is the most favorable 

 of all habitats for plants. Because the plants are of about 

 the same weight as water or are lighter, they are supported by 

 the water and do not need to use their foods for building 

 mechanical tissues. In the shallow water there is sufficient 

 light for photosynthesis, while the plants are protected from 

 the heating and drying effects of the intense sunshine to which 

 land plants in many situations are exposed. An abundant 

 supply of the carbon dioxid and oxygen needed for photo- 

 synthesis and respiration is in solution in the water, and 

 mineral salts sufficient for the needs of the plants are washed 

 in with every rain. Furthermore, the temperature is more 

 uniform than on land, and this permits the processes of growth 

 and reproduction to go on almost uninterruptedly throughout 

 the entire twenty-four hours. The length of the growing 

 season in temperate and cold climates is longer under the 

 water than on land, because the water protects the plants from 

 the sudden changes of temperature to which land plants are 

 subjected in the spring and fall. The shallow-water plants, 

 therefore, live under the conditions most favorable for plant 

 life. Even the deep-water plants like some of the marine algae, 

 although they are under the great disadvantage of having 

 little Hght, gain many advantages through their water en- 

 vironment. 



The environment of the land plant provides a supply of 

 carbon dioxid and oxygen directly from the atmosphere, and 

 mineral salts may be secured from the soil water with which 

 the plants are in contact. But if the plant grows in full 

 sunlight, it is subjected to much more intense illumination and 

 heating than are water plants, and it must withstand the 



