RESISTANCE OF PLANTS 365 



lates in its turn respiration as well as the translocation of organic 

 substances from the leaves. All these processes occur first of 

 all in the lower leaves, which are more senescent and less resist- 

 ant. Hence, drought hastens depletion and the death and 

 desiccation of the lower leaves. The nutritive substances are 

 translocated to the upper leaves and to the growing points, the 

 existence of which is thus supported at the expense of the lower 

 leaves. 



In plants enduring permanent wilting, competition takes place 

 between the various organs for the water reserves as well as for 

 the nutritive substances accumulated previously. In this 

 competition, water moves to the parts of the plant that possess 

 the highest suction tension and greatest resistance of their proto- 

 plasm. The young leaves withdraw nutritive substances and 

 water not only from the older leaves but also from developing 

 fruit. Permanent wilting therefore leads in the earlier growth 

 stages to the abscission of flower buds, to sterility, and, in later 

 stages, to the formation of shriveled grain, called ''windscorch." 

 Windscorch will be the greater, the more vigorous the develop- 

 ment of the leaf surface was up to the time of the beginning of 

 *^he drought. 



The combination of meteorological conditions unfavorable for 

 'he yield is when drought or hot dry winds in the second half of 

 the summer are preceded by a comparatively moist spring and 

 early summer, which contribute to a vigorous development of 

 the vegetative organs. But moderate humidity and even a 

 temporary drought in the first stages of development, which 

 contribute to a less vigorous development of the vegetative organs 

 and besides harden the plants somewhat, similar to the hardening 

 to frost by low temperatures, represent conditions that favor 

 resistance to drought in later stages of development. In such 

 years, arid regions produce their highest yields. 



85. Peculiarities of the Water Balance of Plants in Dry Habi- 

 tats. Xerophytes, Mesophytes, and Hydrophytes.— Water is 

 very unequally distributed over the surface of the earth. There 

 are places where plants always have an ample supply of moisture 

 at their disposal. But there are also large tracts of desert with 

 a very scanty vegetation, a few isolated shrubs scattered over a 

 naked soil. The plants of such dry habitats, where the soil 

 contains but little water and the air is hot and dry, are called 



