EXTERNAL CONDITIOXS OX PLANT LIFE. I 75 



supply. It will be readily understood that at any very high alti- 

 tude the amount of dust in the air is at a minimum; also, that 

 the greater the altitude the less dense the atmosphere, which 

 will, consequently, contain less moisture. But as the illumina- 

 tion of general space depends wholly upon the refraction of 

 light from the dust and particles of moisture in the air, it will 

 be seen that refraction at high altitudes must be at a minimum, 

 and that air spaces outside of the direct line of the rays of 

 lio-ht will be much darker than at lower altitudes. The light 

 given to shaded leaves, then, at these high altitudes through 

 refraction, often proves too little for the purposes of assimi- 

 lation, and the assimilating tissue consequently disappears. 



This shows the relation of altitude to light and assimilation. 

 But altitude is many-sided in its influences. Because we have 

 here less dust and less moisture in the air, the sun's rays, not 

 being refracted out of their course, are much more direct. In 

 the shade, at an elevation of seven thousand feet, you are too 

 cool, but step into the sunlight and your upturned face will be 

 quickly burned by its strong, direct rays. These same rays 

 burn the moisture rapidly out of mountain leaves. To avoid 

 this as much as possible, they become thickened in many ways, 

 often decrease in size, and travel along the same lines 

 generally for water retention as have the desert plants. High 

 altitudes, then, will produce desert-like plants, as seen in the 

 Live-for-evers, or Scdnins. Not only are the leaves found 

 decreasing in size and becoming thickened, but in all sensitive 

 plants the movements are much more rapid. 



Many plants which do not have daily movements of leaves 

 to accommodate themselves to the light, still put their young 

 leaves, when thoroughly exposed to the sun, parallel to its rays 

 in the hottest part of the day. Later in the season, when the 

 epidermal tissues are more thoroughly developed and the cuticle 

 has been fully thickened, the leaves, thus becoming better able 

 to resist the heat rays, drop down to a position more or less at 

 right angles to the approaching light. Select another plant of 

 the same species, which is thoroughly shaded during the whole 

 day — it may not be twenty feet from the first — and we shall 

 find that the young leaves, as they develop themselves, at once 



