516 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



If, therefore, we consider the sensitiveness to light as equivalent 

 to the sense of sight, all green parts of plants possess it; for no 

 plant naturally living in bright sunshine can flourish in darkness, 

 or rarely well even in subdued light. 



The chlorophyll granules utilize the yellow and blue rays espe- 

 cially for the purpose of assimilation of carbon from the carbon 

 dioxide in the air; while the red and violet rays especially, as well 

 as to some extent the green, are required for transpiration. 



The invisible heat rays, which form the continuation of the 

 spectrum below the red, stimulate respiration. 



Lastly, Sachs thinks he has discovered the use of ultra violet 

 rays to be in the process of flower-making, at least in Tropaeolufii. 



Every plant naturally adapted to receive light, theoretically at 

 least, requires an optimum degree for perfect growth and develop- 

 ment. Probably this optimum is never obtained. On the other 

 hand, an excess is common in deserts and a deficiency in forests. 

 Plants have the power to a limited extent to reduce the former and 

 increase the latter. Thus in deserts the leaves are very frequently 

 covered with hairs, forming a sort of felt over the surface, or the 

 cuticle is thickened and corrugated, dispersing some of the incident 

 rays. Some even develop a superficial plate.* 



On the other hand, if the light be insufficient, the superficial 

 cells may take a sub-globular or convex form on the outer side. 

 This, like the eye-ball or magnifying glass, concentrates the rays 

 upon the chlorophyll granules collected at the bottom of the cell. 



Lastly, in many cases, as of the delicate algae, &c., the chloro- 

 phyll grains can move away from the horizontal surface, if the 

 light be too strong, and arrange themselves on the vertical walls, 

 and vice versa when the light is 'feeble. 



Another effect of too great a degree oif light, is to make the 

 green colour to acquire a yellow appearance. This is normally the 

 case with the leaves of Aucuha japonica; for those leaves which 

 are within the bush are of a deeper green and less spotted with 

 yellow than those on the surface. Variegated shrubs, as Holly and 

 Yew, have only the exposed leaves yellow, those below being green 

 like the Aucuha, only in their case it is an acquired habit. 



Another effect of light is seen in the alteration of the position of 

 leaves. These endeavour to place themselves with their blades at 

 right angles to incident light. This can be effected in different ways. 

 Thus, in the common Laurel, the shoots project horizontally from 

 the sides of the bush; these bear leaves in a distichous manner (or \); 

 but on shoots arising vertically from the top of a bush the leaves 

 are scattered (on the fth plan). 



In the Yew, the short undeveloped shoots have the leaves scattered 

 (fth plan) ; but they all lie horizontally on a lateral shoot, by twisting 

 their short petioles. 



Shoots with opposite leaves in pairs can cause all the leaves to face 

 * Schimper's Plant Geography, p. 59. 



