220 



If the same leaves are examined later in the day or at a time when 

 the sunlight is near its maximum intensity, it will be seen that the 

 leaflets are no longer spread out to receive its full force but that they 

 are folded together in pairs in such manner that the edges are presented 

 to the sun, so that the rays strike the surfaces at a very acute angle ; 

 and the entire aspect of the plant is altered. (See Fig. 2.) 



Fig. 2. Closed position of leaf of Pithecolobium Saman Guango to be seen at 

 ' noon-day The position of the leaflets at night is slightly different. 



The meaning of this movement may be most easily interpreted when 

 the functions subserved by the leaves are recalled _ 



For the purpose of this discussion we may consider a green leaf as a 

 mill into whic h water laden with mineral salts from the soil, and carbon 

 dioxide from the air are poured, which converts these substances into 

 sugar and other useful compounds. The driving power of this leaf-mill 

 is sunlight, and the energy is absorbed from the light by means of the 

 green colouring matter (chlorophyll ) Like all mills the leaf does its 

 work best when a certain amount or intensity of force is applied to it, 

 or in other words when it is driven at a certain rate of speed. If 

 driven tro rapidly not only is the work done less perfectly but the 

 mechanism may suffer injury. These facts apply exactly to the leaf. 

 It functions best when it receives light of a certain intensity. 



In the plants which I have mentioned, the leaflets receive the proper 

 amount of light for the successful performance of work when in a hori- 

 zontal position, in the early part of the forenoon. As the sun advances 

 toward the zenith, the heat and light of its rays increase in intensity, 

 and at the same time the rays infringe at right angles on the surface of 

 the leaves held in a horizontal position, greatly increasing their effect 

 on the leaves. If the leaves remain exposed to the full force of the 

 sun's rays in the morning posiiion, the green colour will be destroyed 

 and water will be evaporated faster than it can be brought up from the 

 Toots, and wilting or withering will ensue. 



The leaflets are automatic machines however. The moment the sun 

 beats too fiercely upon the blades, an action is set up which results in 

 turning the edges toward the sun, and the effect of the rays is greatly 

 lessened. In this manner the plant has altr ost absolute control over the 

 amount of heat and light which may be allowed to act upon the leaves. 

 A glance at many plants found in every door-yard will show plants 

 which have made this adjustment to the noonday sun.. 



