306 TEXTBOOK OF PLANT PHYSIOLOGY 



rupted and unhindered access to the growing cells, which is pos- 

 sible only when it is used at a comparatively slow rate. With 

 deficiency of water or an accumulation in the soil of substances 

 inhibiting water supply (Arts. 45 and 46), the stage of elongation 

 terminates too early and differentiation begins before the cells 

 have reached their full size. The result will be a plant with smaller 

 cells and shorter growth. 



A too rapid expenditure of water leads to the same consequences. 

 Leaves that transpire fast cannot obtain water rapidly enough 

 from the soil and will draw it from the growing organs. Under 

 these conditions, the expanding cells will be disturbed most, result- 

 ing in diminished size. The drawing of water by the older leaves 

 from the growing ones may explain the law of Zalensky already 

 discussed in Art. 58, according to which the upper leaves have 

 always smaller cells and show the fundamental xeromorphous 

 structure. This may be chiefly due to an insufficiency of water 

 supply for the growing organs. 



In cereals to which belong the most important crops, such as 

 wheat, rye, barley, and oats, the developing stem shows a peculiar 

 growth distribution. First, only meristematic growth takes place 

 in the stem, and only after all its internodes, as well as inflores- 

 cences, have been laid down, does elongation, or the so-called 

 shooting, set in. During this period, the plant is especially in 

 need of an ample water supply for the growing parts and, hence, is 

 sensitive to its deficiency. Field observations have shown that 

 an insufficient rainfall during the period immediately preceding 

 shooting greatly reduces the yield of cereals. 



Experiments have demonstrated likewise that the same amount 

 of wilting of plants will produce quite different effects if it occurs 

 at different periods of their development. Wilting of tillering 

 plants, i.e., during the period of meristematic growth, or in the 

 time of blooming, coinciding with the stage of internal differentia- 

 tion, is less detrimental than wilting at the time of shooting. Hence, 

 in respect to moisture supply, this stage of development is known as 

 the critical period (Brounov). 



The most favorable conditions for growth are related to an 

 ample moisture supply in the soil as well as the atmosphere during 

 all periods of development of a plant. Consequently, the greatest 

 vigor and the highest rate of growth are attained by the vegetation 

 in the moist and warm tropical countries. A constant heavy 



