234 TEXTBOOK OF BOTANY 



absorption there will be no further enlargement. Likewise, any increase 

 in size of other parts of a plant occurs by cell division and enlargement 

 or by the enlargement of cells previously formed. Consequently, the 

 growth of all plant organs may be affected by transpiration. Further- 

 more, the rates of chemical processes (oxidation-reduction, hydrolysis, 

 and condensation ) may vary as the water content of the cells fluctuates, 

 and result in changing the chemical compounds formed in these cells. 

 Some of the effects of drought upon the growth of the epidermis and 

 mesophyll of leaves are illustrated in Chapter IX. 



The effects of low water content in plants are most pronounced dur- 

 ing those occasional short or prolonged periods of drought, of high tem- 

 perature, or of low humidity, when transpiration greatly exceeds water 

 absorption in many species of plants. If a number of different species 

 are exposed to these extreme environmental conditions, the effects on 

 their individual rates of transpiration, their photosynthesis, their growth, 

 and their survival are usually different. The vegetative tissues of certain 

 fungi, mosses, ferns, and a few seed plants may become dry and 

 brittle without being killed. They may survive in this condition for 

 days or weeks, and resume growth when water becomes available. 

 The vegetative tissues of most plants, however, die from the effects of 

 excessive water loss long before they become air dry. A deficiency of 

 water within the cells of plants is sometimes referred to as "internal 

 drought." 



Transpiration from seeds, bulbs, tubers, and other dormant organs is 

 slow, and these are the only organs of many plants that survive prolonged 

 periods of drought. 



The very slow-growing and succulent species of desert regions and 

 other extremely dry habitats may survive, grow, and reproduce in sur- 

 roundings in which other species wither and die in a short time. Experi- 

 ments have shown that some of these succulents, when detached from 

 the soil and placed on a window ledge or table, not only survive for 

 months, but may even bloom and bear fruit several months later. The 

 extremely slow rate of transpiration from these plants in a hot desert 

 climate is almost incredible. 



Some of the differences in rates of transpiration among a variety of 

 species of plants growing in diverse environments, are summarized in 

 Table 8. 



On an average day in the growing season a mature corn plant may 



