138 PLANT PHYSIOLOGY 



correlation may be observed between the concentration of the 

 cell sap and growth. This is especially obvious in the growth of 



tree shoots. 



In spring, the root system delivers abundant water, and, 

 evaporation being insignificant, there results a vigorous growth 

 of the shoots. With the development of the leaf surface, 

 evaporation begins to overtake the delivery of water, the concen- 

 tration of the cell sap increases, and growth first decreases and 

 then stops. In general, with a deficiency of water or with 

 accumulation in the soil of substances that inhibit the water 

 supply, the stage of elongation terminates too early, and differ- 

 entiation begins before the cells have reached their full size. This 

 leads to the production of small cells and stunted plants. 

 The same result is obtained when the water loss is too rapid. 

 Strongly evaporating leaves may lose water faster than it is 

 dehvered from the soil; consequently, they withdraw water 

 from the growing organs and disturb most of the tissue in the 

 zone of elongation, and this leads to the formation of small cells 

 and to a diminution in the size of the entire plant. 



In annual cereals, to which belong our 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 inflorescences, have been laid down, does elongation or the 

 so-called ''shooting" begin. During this period, the plant is 

 especially in need of an ample water supply for the- growing 

 parts, and, hence it is sensitive to any deficiency. Field observa- 

 tions have shown that an insufficient rainfall during the period 

 immediately preceding shooting greatly reduces the yield of 

 cereals. That is why this growth phase has been designated a 

 critical period in relation to moisture (Brounov). 



The most favorable conditions for growth are an ample mois- 

 ture supply both as soil water and atmospheric humidity during 

 all periods of development of a plant. Consequently, the 

 greatest vigor and the highest rate of growth are attained by 

 vegetation in warm, moist, tropical countries. 



Artificial irrigation applied in many arid regions, e.g., in the 

 western United States and in Middle Asia, exerts a great influence 

 \ipon growth. Irrigated plants are always considerably higher 

 than nonirrigated ones, develop a larger leaf surface, and corre- 



