112 ENVIRONMENTAL CONDITIONS. 



into all enlarging portions of the plant-body. This demand for water, 

 due to growth, is usually quite insignificant in degree, but it is never- 

 theless real, and the absence or inadequacy of the water-supply to 

 growing cells must always act as a check upon enlargement. 



Also, since water is destroyed in the process of photosynthesis in 

 green plants, there must be a continuous influx of moisture into all 

 photosynthetizing cells. If this supply is cut off, the formation of 

 carbohydrates must cease in a short time. With insufficient water- 

 supply, growth ceases before photosynthesis, but both processes must 

 soon be brought to a standstill. The water demand occasioned by 

 photosynthesis is perhaps usually more pronounced than that for 

 enlargement, yet the rate of influx thus brought about is far too 

 small in amount to permit of measurement by simple methods. Also, 

 water disappears when hydrolytic decompositions occur, so that such 

 processes as the digestion of starch are essentially drying processes. On 

 the whole, these water requirements may be safely assumed to be only 

 of relatively slight importance in comparison with that of transpiration, 

 which is next considered. 



The fact that all plant tissues contain water, and that no cuticular 

 or other covering is absolutely impervious to this liquid — many leaf- 

 cuticles, etc., being rather freely permeable to water — makes it 

 logically follow that there must ever be a more or less pronounced 

 evaporation of water from all plant surfaces that are exposed to the 

 outer air. This superficial evaporation through externally exposed 

 membranes makes up the so-called cuticular transpiration, which 

 varies in amount in different forms, depending upon structure, and is 

 often of great importance in determining the need of the plant for 

 water. The water lost by cuticular transpiration is replaced from 

 more deeply-lying tissues, according to the principles of diffusion and 

 of imbibition, and sooner or later there must occur an inward move- 

 ment of water from some region without the plant-body, or else death 

 must ensue. 



But, while cuticular transpiration is a very real and almost con- 

 stant source of water requirement, it is of relatively little account in 

 comparison with stomatal transpiration. The presence of mem- 

 branes of high moisture-content within the leaves — the walls of the 

 mesophyll, etc. — which are in direct connection with the external air 

 through the stomata, -makes continuous evaporation from these 

 internal tissues an inevitable condition, unless, indeed, the foliar 

 surface be covered with a film of water or of a solution of higher vapor- 

 tension than that which occurs within the tissues. The internal 

 atmosphere is maintained more or less nearly in moisture equilibrium 

 with the wet membranes that bound it, and ordinary diffusion through 

 the stomatal pores constantly removes water-vapor to the outer air. 



