14 BOTANY OF THE DEATH VALLEY EXPEDITION. 



access to the conditions of heat, light, moisture, food, and air, and yet, 

 from lack of a place iu which to stand, be unable to grow. The ab- 

 stract condition that we may call position is included, therefore, under 

 inechanique as one of the necessaries of life in the plant. Further- 

 more, it has been ascertained by comparatively recent experiments 

 that the minute mechanical structure of the soil has a great effect upon 

 the accompanying vegetation; that, for example, a sandy soil will sup- 

 port a plant which, in a clayey soil containing precisely the same food- 

 materials and moisture and exposed to the same conditions of heat, 

 light, and air, would perish. This purely mechanical influence of the 

 soil has been found to have a marked effect in varying the vital 

 energy of plants, and therefore in deciding which species shall fail, in 

 certain places, to come to reproductive maturity. The fact that a wide 

 variation exists in the mechanical structure of soils, therefore, renders 

 the condition that is here named inechanique a, very important con- 

 sideration in plant distribution. Other examples of conditions that fall 

 under this head may be cited, such as heavy winds and the eating of 

 vegetation by animals. 



Having now named what are here considered the ultimate conditions 

 under which plants are developed and modified, it remains to enum- 

 erate and analyze some of the ordinary conditions which, as stated 

 above, may be treated as compounds of these simple factors. 



A glance at a phytogeographic map of the earth — such, for example, 

 as that published in Berghaus's Physikalischer Atlas — indicates that 

 differences in latitude are of primary importance in the geographic 

 distribution of plants. The floral regions there published conform 

 in general with latitudinal lines, and the influence of this variation in 

 latitude is due, of course, preeminently to differences of temperature 

 expressed in the seasons. 



It should be pointed out here that the mean annual temperature of 

 a place is not necessarily a close index of the thermal relation of that 

 place to its flora; that is, two points having tin 1 same mean tempera- 

 ture may support quite different floras, from the fact that one of them 

 has a very warm summer and a very cold winter, while the other has a 

 much more equable temperature. Variations in the temperature of the 

 hibernation period, especially in regions whose winter heat is rarely 

 above 32° F., has little effect upon the vegetation. The mean tem- 

 perature for the season of growth is therefore a nearer approximation 

 than the mean annual temperature. It often happens, furthermore, that 

 of two places showing the same mean summer temperature, the one has 

 this season prolonged a month or two more than the other. This extra 

 time, which is not expressed in the mean summer temperature, may 

 permit the presence of many species of plants which in a shorter season 

 would fail to mature and would soon become locally extinct. It has 

 been proposed, therefore, to measure the potential growth of a season by 

 summing up the daily temperatures. The resultant sum has been called 





