86 APPLIED BIOLOGY 



some similar plants for illustration and try some experiments 

 with plants which are so much like the bean plant that we 

 have good reason to believe that the work of their organs is 

 the same. It should be remembered, then, as we proceed 

 with the study of plant physiology that the processes or 

 functions being studied are essentially the same in all plants 

 which have organs such as we have found in the bean plant. 

 Preview of plant life. In order to carry on their life- 

 processes, plants must have food, water, and oxygen, as ani- 

 mals do. Some land plants get the first two of these essen- 

 tials entirely from the soil and others in part. The oxygen 

 is absorbed chiefly from the air through the leaves. Water 

 from the soil enters the roots and ascends to the leaves, 

 carrying up certain food-materials. In green plants certain 

 foods are formed in the leaves from water and carbon dioxide 

 from the surrounding air. Sap containing foods made in 

 the upper part of a plant may flow down certain tubes in the 

 stem. The details of these processes will now be considered. 



82. The Need of Water. In one of the first experiments 

 we found that plants contain a large amount of water. 

 Moreover, any one who has ever cultivated plants knows 

 that unless the soil is kept moist the plants will wither and die. 

 Evidently water must be of great importance; and so it 

 will be of interest to study (1) how water gets into the plant 

 organs (root, stem, and leaf) and (2) what work water does 

 in these organs. 



83. Source of Water. It is obvious that ordinary plants 

 which have roots must get most of their water from the soil. 

 It might be supposed that some water from rain and dew 

 which wets the leaves is absorbed ; but that this is exceed- 

 ingly small in amount and insufficient could be proved by 

 taking a potted plant and covering the soil with waterproof 

 cloth so that rain and dew touch the stem and leaves but 

 not the roots. Under such conditions most kinds of ordinary 

 plants would soon wither. 



