336 PLANT PHYSIOLOGY 



dinarily produce the linear leaves, develop into long-petioled, cor- 

 date leaves like those found normally only near the ground. 



Moisture. — Water is absolutely necessary in the plant. It plays 

 an important direct or indirect role in the various chemical reac- 

 tions which take place within the plant, and it also provides 

 turgor, which is a necessary physical condition. Directly, water 

 takes part as one of the reacting substances in photosynthesis 

 and in the various hydrolyses which take place within the organism. 

 All the other reactions occur in a watery medium so it should not 

 be necessary to emphasize further the part played by water 

 (Chap. XX). 



On the basis of their water requirements, plants are divided 

 into three categories: hydrophytes, mesophytes, and xerophytes. 

 The first named include such plants as water lilies (Nymphaea), 

 Elodea, Utricularia, the algae, etc., which require a great abun- 

 dance of water. The mesophytes include the average plants, which 

 get along on a medium amount of water. Xerophytes are built 

 in such a manner that they can thrive, or at least live, on a very 

 small water supply; they include the cacti, yuccas, and other 

 desert plants (Chap. XX). Some of the so-called mesophytes are 

 really tropophytes, i. e., they alternate between a hydrophytic 

 condition in the spring and summer and a xerophytic one in the 

 fall and winter when water is not available. There are also physio- 

 logical xerophytes or halophytes found particularly in salt marshes, 

 alkali regions, brackish streams, etc. These plants are living in a 

 region where water is abundant, but, owing to its high salt content, 

 they find it difficult to absorb. They hence often assume the 

 fleshy, succulent appearance of many xerophytes. 



It is possible for plants to get too much water especially at the 

 fruiting season, when the fruit may take up so much water that 

 it bursts and becomes unfit for sale. Water is also harmful to 

 many plants like the date if it comes at the season of pollination, 

 when the pollen grains, gorged with sugar, become so full of water 

 because of their osmotic concentration that they burst and are 

 valueless. The same damage may be caused to the pollen of 

 many of the common fruit trees. 



Water also exercises a strong morphogenic influence. Amphib- 

 ious plants like Proserpinaca, which grows partly in water and 

 partly exposed to the air, have finely dissected leaves in the water 

 and flat, undivided ones in the air. The height at which the flat ones 



