CONCLUSION AND SUMMARY 65 



root hairs in times of excessive wilting. In nature a shower 

 which revives the plant but does not furnish sufficient water 

 to the soil for its growth will not enable it to push out new 

 root hairs, and as a result only permits the plant to live 

 longer by retarding excessive evaporation. 



The ability of plants to take water from the soil varies in 

 an ascending scale from hydrophytes, through mesophytes, 

 to xerophytes The limit of available water for a plant, in 

 a general way, coincides with the physical conditions of its 

 habitat. This being the case we have variations not only 

 among genera but also among species, and even among indi- 

 viduals, depending upon the physical conditions under which 

 the plant has grown and upon tendencies inherited from its 

 ancestral forms. The relative activity of the protoplasm is 

 a very important factor in determining the amount of water 

 that a plant may abstract from the soil in times of drought. 



C. DROUGHT EFFECTS. 



Herbaceous plants, both annuals and biennials when 

 affected by slow drought usually die in the following man- 

 ner. The older leaves of the plant perish first in order of 

 their age, the younger leaves living to the last; the finer 

 roots die first, then the coarser roots, finally the stem and last 

 of all the growing point of the stem, or if the plant is flower- 

 ing, the immature fruit. If a plant of either type has suf- 

 fered a structural injury, it is very apt to die from the point 

 of injury, since it loses water more rapidly at this point. 



Trees and shrubs and pleiocyclic herbs lose their aerial 

 organs first, the roots being usually the last part of the 

 plant to die. Bulbous and tuberous plants although imma- 

 ture form bulbs or tubers rapidly when caused to die by 

 drought. Propagation has become the favorite means of 

 continuing the existence of these plants. 



The structure of the protoplasm of , the meristem cells in 

 the growing point of the stems and in the immature fruits of 

 herbaceous plants, as well as in others, must be of such 

 chemical and physical nature as will enable these cells by 



