16 BOTANY OF THE DEATH VALLEY EXPEDITION. 



of a plant's environment combines a greater number of primary in- 

 fluencing- conditions than this, and none within the limits of a plant 

 zone appears to have a more direct influence upon plant life, An 

 important part of the raw food-material of a plant must be absorbed 

 from the soil, the amount contained therein varying with that in the 

 rock from which the soil was formed and with that afterward infil- 

 trated from other sources. A sufficient, yet not too great, supply of 

 water must be furnished the plant, and in the control of this supply 

 the soil almost solely is concerned.* This phase of the influence of soil 

 has been made the subject of a detailed study, under the auspices of 

 the Department of Agriculture, by Mr. Milton Whitney, 1 and the re- 

 sults obtained by him find expression quite as fully in natural vegeta- 

 tion as in cultivated plants. Two secondary influences are dependent 

 upon the amount of water in the soil, namely, the temperature of the 

 soil and the amount of contained air. The lack of a sufficient amount 

 of soil air prevents the proper aeration of the roots and injures or kills 

 the plant. 



To sum up, the six ultimate factors in the distribution of vegetation 

 are heat, light, water, food, air, and mechanique. These factors are 

 variously combined in actual fact into such conditions, among others, 

 as geographic isolation, latitude, altitude, rainfall, soil, fires, proximity 

 to large bodies of water, slope exposure, and presence of forests. 



The external conditions of plant life, which have been briefly dis- 

 cussed above, constitute one of the two groups of forces which have 

 combined to bring about the present distribution of plants. This first 

 group of forces is the environment, the second is the plants themselves 

 with all their vital energies. The different species possess certain 

 structural and physiological characteristics, such as mechanisms for 

 the transportation of seeds and ability to withstand freezing, allot 

 which occur in endless variety. The consideration of this second set 

 of forces makes up a large part of textbooks on physiological and 

 structural botany, and no analysis of them need be given here. 



COMPARATIVE VALUES OF PLANTS IN DETERMINING FLORAL ZONES. 



The term zonal plant as used in this report, designates a species or 

 variety which is of value in determining floral zones. Not all plants 

 are serviceable for this purpose, as a considerable amount of field work 

 has clearly demonstrated. Upon the Death Valley Expedition it was 

 found impracticable, after a short experience, to record each day in the 

 field a series of observations including all the altitudes of all the species 

 seen, and at the same time to collect specimens of eacli new or doubtful 

 plant, rt was found to be the best method of procedure in a new area 

 .to establish the zones by means of a comparatively small number of the 



1 Some Physical Properties of Soils in their Relation to Moistnrc and Crop Distri- 

 tration. By Milton Whitney. U. S. Department of Agriculture, Weather Bureau, 

 Bulletin No. 4. 1892. 





