198 INFLUENCE OP. LIGHT UPON LEAVES AND STEMS. 



before they expanded. The following is an example of this fact, 

 more remarkable than is ever seen in this country. *' It fre- 

 quently happens in America, that clouds and rain obscure the 

 atmosphere for several days together ; and that, during this time, 

 the buds of entire forests expand themselves into leaves. These 

 leaves assume a pallid hue until the sun appears ; when, within 

 the short period of six hours of a clear sky and bright sunshine, 

 their colour is changed to a beautiful green." One writer men- 

 tions a forest on which the sun had not shone for twenty days. 

 " The leaves during this period were expanded to their full size, 

 but were almost white. One forenoon, the sun began to shine in 

 full brightness ; the colour of the forest absolutely changed so 

 fast, that we could perceive its progress. By the middle of the 

 afternoon, the whole of this extensive forest, many miles in 

 length, presented its usual summer dress." 



309. The influence of light is also shown, in modifying the 

 direction of the stem. Where a plant sends forth a single stem 

 or shoot, it will always direct itself towards the light ; and this 

 is especially manifested, where the light comes in only one direc- 

 tion, as when a Potato, which has begun to grow in a cellar, 

 sends a shoot of several feet in length towards any aperture, 

 through which even a small quantity of light finds admission. 

 The reason obviously is, that, in consequence of the loss of fluid 

 from the tissue of the stem, on the side on which the light falls, 

 it is contracted, whilst that of the other side remains turgid with 

 fluid ; the stem makes a bend, therefore, until its growing point 

 becomes opposite to the light, and then increases in that direction. 

 Should any obstacle divert it, the same cause will bring it back. 



310. The direction of the branches of trees growing in open 

 spaces, is less influenced by light ; since the rays of the sun exert 

 a much inferior power in this respect, when they are reflected 

 from clouds and other objects, so as to form what is commonly 

 termed " diffused daylight." It is a curious fact, however, that 

 there seems a tendency in almost every growing tree, to send its 

 principal trunk directly upwards. Experiments have been made 

 for the purpose of changing this. For instance, the stem has 

 been bent much out of the perpendicular, by means of a rope 



