3. Transpiration may be checked by reducing the nmnber of leaves. 

 If you can recall any plant, say a wild rose, and compare one growing in 

 moist soil with one gro'Bdng in dry soil, you will at once see how often 

 nature makes use of this device to prevent damage by excessive tran- 

 spiration and to fit the plant to meet its conditions. And in this way 

 also, the leaf tells us of the water content of the soil. You know 

 farmers and gardeners say that in wet weather their plants all 

 run to leaf, which only means that no check need be placed upon tran- 

 spiration. 



3. Transpiration may be checked by thickening the outer wall of the 

 leaf. If you compare a leaf of a plant growing in dry soil with that of 

 one growing in very moist soil, the former will in almost every case 

 have the thicker and tougher outer covering. This is one of nature's 

 favorite devices for checking transpiration, and you can scarcely ex- 

 amine a leaf taken from a plant growing in dry soil which will not 

 show it and at the same time tell to you the character of the soil as to 

 its water capacity. 



4. Transpiration may be checked by the leaves having a covering 

 of hairs. This also is of frequent occurrence in nature. The common 

 mullein is a familiar example of this method of controlling transpira- 

 tion. This of course is not the only use of hairs, as may be shown in 

 some future leaflet, but it is one of their important uses. 



There are other methods of checking transpiration, but we are 

 only concerned with those which are readily apparent and can be used 

 in nature work. 



If we compare then the foliage leaves of plants growing under dry 

 conditions with those of water-loving plants, the following facts are 

 apparent: 



1. The leaves are relatively small. 



2. The leaves are often fewer in number. 



3. The outer covering of the leaf is thicker. 



4. The leaves are often clothed with hairs, which in water-loving 

 plants are almost always wanting. 



The intermediate plants show almost all conceivable variations be- 

 tween these extremes and are extremely sensitive to the slightest 

 changes in soil and air moisture, recording- these changes in cor- 

 responding leaf modifications. The difEerences in many eases in plants 

 of the same species growing under difl'ering conditions is so marked 

 as to have led to the formation of distinct species, when the plant was 

 merely trying to tell us the story of the soil. 



It is not wise, in these studies, to press the work upon a single 

 feature too far. Continued application is an acquirement of age. The 



