74 BOTANY: PRINCIPLES AND PROBLEMS 



into another or with the breaking down of complex organic 

 compounds into simpler ones. Photosynthesis alone is funda- 

 mentally constructive, and the activity of green plants thus 

 underlies that of all other organisms. 



Transpiration. — The lower portion of the mesophyll, or the 

 spongy layer, is not concerned primarily with photosynthesis 

 but with the interchange of gases between plant and atmosphere. 

 Notable among these interchanges is the evaporation of water 

 from the tissues and its passage into the air, a process which we 

 know as transpiration. 



The Importance of Water. — The water-relations of a plant are 

 of the utmost importance to it and profoundly influence its 

 structure and activities. We have seen that water constitutes 

 the major portion (75 to 90 per cent) of plant tissues in general, 

 and a very much larger share of protoplasm itself. An abun- 

 dance of water keeps the cells plump, and by maintaining the 

 turgidity of the tissues, enables the soft parts of the plant to 

 preserve their firmness and to function successfully. Water is 

 one of the raw materials entering into the process of photosynthe- 

 sis. It is the solvent of the mineral nutrients, which can enter 

 the plant and move about within it only when in solution, and 

 in watery solutions all the important physiological processes of 

 the plant take place. The maintenance of an abundant supply 

 of water in its tissues is therefore essential for the life and growth 

 of the plant. 



To this end the primary requisites are evidently the presence 

 of a sufficient amount of available water in the soil and its 

 abundant absorption therefrom by the roots. Of no less signifi- 

 cance in the water-relations of the plant is the process by which 

 this water evaporates from the plant tissues and passes into the 

 air. Absorption must equal or exceed transpiration if the plant is 

 to thrive, for should there be a deficiency in income or an excess 

 of outgo, a shortage of water will result in the tissvies, and the 

 plant will suffer accordingly. 



Only a very small fraction of the water which enters the root- 

 hairs and passes upward to the leaves takes part in the manu- 

 facture of sugar. The remainder becomes distributed through 

 the cells of the spongy layer and evaporates from their moistened 

 walls, departing through the stomata as water vapor (Fig. 38). 

 A smaller amount may be evaporated directly from the surface 

 of the epidermal cells. During the growing season a constant 



