18 BOTANY OP THE DEATH VALLEY EXPEDITION. 



this fact. Shrubs and trees, being commonly larger than herbaceous 

 plants, reach higher into the air and penetrate more deeply into the 

 soil, thereby subjecting themselves to a wider range of conditions than 

 do these smaller plants. They also, by continuing throughout the year 

 exposed to successive, varying seasonal conditions, complete the full 

 round of their possibilities in environment. They therefore stand as 

 the most complete summation that can be attained of the natural light, 

 heat, moisture, food, air, and mechanique of any area; in other words, 

 a sure index of the natural agricultural capacity of the soil upon which 

 they grow. From a utilitarian point of view, too much stress can 

 scarcely be laid upon this fact. It has been the practice of agricultur- 

 ists to gauge the capacity of soils, in regions new to the plow, by ob- 

 servations on rainfall, temperature, cloudiness^ chemical composition of 

 the soil, drainage, and many other j>henomena, or by the even more 

 laborious process of experimenting on every farm with each kind of 

 cultivated product; ignoring the fact that this determination can be 

 greatly hastened, cheapened, and authenticated by correlating the 

 natural vegetation, especially that made up of the trees and shrubs, 

 with that of other regions whose agricultural capacities are known. 



Perennials stand next in value to woody plants and are constantly 

 used in zonal classification. They differ from the last group in their 

 usually smaller size, and especially in the death, each autumn, of that 

 portion of the plant above ground. This fact removes from the ex- 

 periences of perennial plants the aerial conditions of the winter 

 months, and to this extent they fail to represent the total of their pos- 

 sible environment. The effect of winter climatic conditions upon the 

 life of a plant is really of slight importance in comparison with that of 

 the conditions active during the period of growth and reproduction, 

 yet the effect of the former is often clearly apparent. 



Least reliable of the three vegetative groups of plants are annuals, 

 for they represent the conditions of but a single season of growth. 

 The importance of this fact is clear when the wide variation in condi- 

 tions from year to year is considered. A season of unusual cold, or 

 great drought, or extraordinary rainfall may completely change the 

 aspect of the annual vegetation of any district, and those plants which 

 one year indicate its character may entirely fail to do so in the year 

 following. It is found furthermore that the vertical range of these 

 plants is very uncertain. If, for example, a small annual, the length of 

 whose life is but two months, is accustomed to grow upon hot sandy 

 soil, it is very likely to find during an unusually warm summer con- 

 genial spots at altitudes far above its ordinary range, in which it can 

 grow and bear seed abundantly. This could not happen to a perennial 

 or a shrub, for before such a plant reached the maturity necessary for 

 reproduction, adverse conditions would destroy its life. 



For the reasons given above, therefore, trees and shrubs must form 

 the basis of zonal work. That they are of such importance is the more 



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