12 BOTANY OF THE DEATH VALLEY EXPEDITION. 



means for mutual transportation of seeds, by the agency both of wind 

 and of animals; but the mingling of the two floras under the present 

 climatic conditions is impossible, for neither can the species adapted to 

 the moisture-laden air of the valley exist in the exceedingly arid atmos- 

 phere of the desert, nor can the desert plants compete with those of the 

 valley under conditions favorable to the latter. 



CONDITIONS THAT AFFECT DISTRIBUTION. 



The continued existence of a species requires: first, that its individ- 

 uals grow; and secondly, that they reproduce. That a species may 

 spread requires, in the third place, that its reproductive bodies be dis- 

 seminated. Under these three heads — growth, reproduction, dissemina- 

 tion — may be grouped all the causes that affect distribution; but since 

 some of these causes would necessarily be duplicated under each head, 

 they will not be so classified, but will be treated in one group. 



Plant growth depends upon the factors of heat, light, water, food, 

 air, and mechanique. These factors usually do not occur in nature in 

 what may be called a pure state, but variously compounded into in- 

 fluencing conditions, such as climate and soil. We will attempt later 

 to take up a few of these compound conditions one by one and reduce 

 them to the elemental conditions enumerated above. First, a short 

 examination of each simple condition will be made. 



The effect of heat upon plants varies greatly with different species, 

 different organs, and different stages of growth. The general important 

 fact is, that for each function of the plant a particular temperature is 

 most favorable, a higher or lower one less favorable, and a still higher 

 or lower one fatal to the performance of that function. Thus each 

 species has, during its period of growth, tenqjeratures known in plant 

 physiology as minimum, optimum, and maximum. Below the minimum 

 and above the maximum, growth ceases; while from these points to 

 the optimum, growth increases, and at the latter temperature is greatest. 

 The principal effect of proper temperature is evaporation of moisture 

 from the surface of the plant, which assists in the circulation of the 

 juices and, by some method not yet understood, in the activity of the 

 protoplasm. A minimum, or lower, temperature lessens evaporation, 

 thereby causing stagnation of circulation and diminution of food sup- 

 ply, and either renders the protoplasm inactive or kills it. A. tem- 

 perature above the maximum is likely to so increase evaporation that 

 the supply of moisture from the roots is not enough to meet the defi- 

 ciency, and the tissues become dry and dead, while the protoplasm often 

 is killed. Besides the period of growth, all perennial plants have a 

 period of rest, the principal external condition of which in extra- 

 tropical regions is a temperature below the minimum for the species. 

 During this period a plant is least susceptible to bad effects from low 

 temperatures, and iudeed requires such temperatures to insure its per- 

 fect health. 



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