250 The Ohio Naturalist. [Vol. XIV, No. 4, 



quantity of water is exceeded. The entire structure of land plants 

 inhabiting dry climates shows the resistance to transpirational 

 water loss and how far such limitations may go; and the plants 

 possessing special body features for accelerating transpiration or 

 for exuding water where transpiration is out of the question, 

 indicate how fundamentally important is the maintenance of the 

 water balance within the plant. Artifical defoliation, an increased 

 water supply or decreased transpiration are known to affect in a 

 number of trees the thickening of cell walls during the formation 

 of autumn wood ; this is caused partly by inferior nutrition, largely 

 by the increased amount of water in the plant. The dearth of both 

 exact knowledge and laboratory experiment make it impossible 

 to state the amount of water involved in hydrolytic reactions and 

 necessary as a constant quantity in the plant during its life cycle 

 for vegetative or reproductive growth. 



It will be seen from the brief remarks above that the rate of 

 growth, the amount of it and the final size attained by a plant 

 depend in part on favorable conditions of temperature, light in- 

 tensity, food supply, and on the amount of water present in the 

 plant. The rate or the total amount of water transpired gives no 

 indication as to the quantity which normally is required for meta- 

 bolic processes and for growth. Moreover, the chemical reac- 

 tions associated with the growth of cells throughout the forma- 

 tive phase, the phase of enlargement of cells and that of matura- 

 tion, by which food inaterials and other substances become in- 

 corporated into body tissue, are largely dehydrating in character. 

 At the growing point it is chiefly a local production of originally 

 combined water set free by dehydration processes and by respira- 

 tion rather than the transpiration water which induces turgor and 

 the elongation of new cells. Many plants, aside from those carry- 

 ing water in a special storage tissue, are able by means of dehy- 

 drating processes to withstand long periods of drought without 

 pemianent injury; and numerous cases are known of fruits, seeds 

 and severed portions of hving plant tissue which are able to main- 

 tain a certain quantity of intracellular water in this manner 

 indefinitely, and for some time a constant loss of water incurred 

 through transpiration. 



It would certainly be quite wrong to conclude that transpir- 

 ation is not essential to plants, merely because it is not directly 

 related to absorption and translocation of solutes, to green and 

 dry weight of plants, and not a measure of metabolism and growth 

 or vegatative luxuriance. The quantity of transpiration water 

 in most plants is certainly not co-ordinated with or related to 

 these functions. The retention of water is the physiological 

 function indisijcnsablc to growth in general, and to sur\'ival 

 and greater areal distribution in regions of a continental climate. 

 But there can be no doubt that transpiration is indicative of 



