DISEASES OF THE PINE. 185 



under these conditions being more active. In this respect, light affects 

 plants to such a degree that even passing clouds will lessen the evapora- 

 tion. The result of all the observations thus far has been to show that 

 under like circumstances the transpiration is greatest in the direct light 

 of the sun — that is, less in common day light, still less in the shade, and 

 least in the night. Risler found by his investigations ^ that in the lucerne 

 the amount of water evaporated in the sun is four times greater than 

 it is in the shade. The difference of evaporation in the two conditions 

 is with this plant considerably greater than with corn. In some plants, 

 as in the willow, it is, however, very slight. This is, no doubt, the reason 

 why some plants will thrive better in the shade than others. Transpira- 

 tion is also diminished by a fall of temperature, and an increase in the 

 humidity of the atmosphere. With the decrease of warmth, and the 

 lessened influence of light, the transpiration of plants becomes less in 

 autumn, and finally stops entirely, causing the falling of the leaves. 

 The evaporation from the leaves is very slight in a damp or foggy atmos- 

 I>here, and when the leaves are wet by dew^ or rain. In the damp air of 

 our hot houses, and under glass vases, often i)laced over weakly plants, 

 the amount of evaporation is very slight. It is correspondingly lessened 

 in the shade of trees, in the cool and damp air of dense forests, and 

 under artificial coverings. 



In order that the leaves of plants may remain fresh and plump, as 

 much water must be taken up by the small fibers of the roots as is lost 

 by transpiration. A constant circulation of water is going on from the 

 roots through the trunk to the branches, and through these and the 

 stems into the leaves. The plant remains in a normal condition when- 

 ever the supply of water by the roots and loss by evaporation corre- 

 spond. Under some circumstances it will occur that the supply of 

 water received through the root is greater than the loss through the 

 leaves, or that the loss is greater than the supply. Instances of the 

 former case are presented in the plant which during the night evap- 

 orates less water than it receives from the ground through the roots. 

 The surplus is deposited on the leaves in small drops, which, upon ex- 

 amination, may be found early in the morning, even in hot-houses, 

 which precludes the idea that they are gatherings of dew. Another 

 instance is shown in our deciduous trees in autumn after the fall of the 

 leaves, when, from a relatively warm soil, the roots maintain their ac- 

 tivity, and continue to receive moisture from the soil, which will remain 

 in the body of the tree, as the organs of evaporation are gone. This 

 explains the reason why there is a greater amount of water in the body 

 of a tree in autumn than there is in summer. It is oftener the case, 

 however, that the amount of water lost iis greater than that received, 

 which occasions in herbage and young plants a withering of the leaves. 

 Larger trees are not materially affected by this interruption, as the body 

 of the tree acts as a reservoir of water, from which the leaves are sup- 

 plied for some time. The withering and drying up of plants is not 

 always the result of an insufiicient amount of moisture in the soil; but 

 it may occur when, in consequence of a lack of activity in the roots, 

 the absorption of water from soil is not proportioned to the loss by 

 transpiration.^ 



The activity of the roots is influenced chiefly by the temperature of 

 the .earth. In a warm soil this, and consequently the absorption of water, 



* Der Katurf or seller, Fobrunry, 1872, p. 45. 



* Plants will dry up Avben the number of small fibers of the roots is too small in pro- 

 portion 1 1 tbo amount of leaves — a frequent occurrence in tho transplanting of large 

 plants when their roots are injured in taking up. 



