DIFFERENCE OF ACTION ON THE UPPER AND UNDER FACE. 



in the camera obscura, a photogenic copy is obtained, every line being represented with 

 mathematical accuracy and the utmost sharpness; the radiation, having impinged upon 

 the plate, does not spread itself out laterally or undergo a process of conduction, but 

 only produces its effect on the portions which lie directly in the lines of light. With 

 radiant heat it is very different: a beam of this agent falling upon a limited portion of 

 a metallic plate, slowly affects all the rest, for the resulting rise of temperature is propa- 

 gated laterally from point to point, and by-and-by the whole mass undergoes an elevation. 

 While light and the tithonic rays, therefore, are not liable to conduction, the reverse 

 holds in the case of heat. Applying these reasons to the instance before us, the rays 

 of light which control the digestive operation could only act in an imperfect way on 

 the under side of a horizontal leaf; they must produce their maximum effect on the 

 surface exposed, and operate elsewhere through imperfect translucency, finding their 

 way through short distances, because the tissues are not perfectly opaque. But the rays 

 of heat, which, with the light, fall on the upper face, are conducted with facility, or 

 transfused by warming the watery juices which are circulating through the organ. 

 Whatever changes, therefore, take place abruptly in the quantity of the incident rays, 

 as when clouds are suddenly crossing the sun, are not abruptly felt upon the stomata, 

 for the leaf, thin as it is, acts as a regulator, and, from the heat which it contains, sup- 

 plies the momentary defect ; the ascending sap also, which is coming through the pet- 

 iole, is coming at an elevated temperature, for, in the course of its ascent from the roots, it 

 has been warmed in its passage through the stem which is enveloped in its dark-colour- 

 ed bark a colour well adapted to receive the effects of the rays of heat. There is, 

 therefore, a reason why the stomata are secreted in the shade, and why the seat of the 

 digestive action lies on the upper face of the leaf. 



410. Besides these well-marked differences of action produced by the rays of light 

 and of heat conjointly in the sunbeam, there are specific effects produced by the vari- 

 ous coloured rays ; thus, the yellow seems to control digestion, and, as Dr. GARDNER 

 has proved (Phil. Mag., Jan., 1844), the blue ray motion. Investigations of the dif- 

 ferent effects produced by the other rays of light, such as the green, the violet, the red, 

 or by those other principles which are invisible ; the phosphoric or the tithonic rays 

 can scarcely yet be said to have been made. Each of these comprises rays differing in 

 constitution, and differing in refrangibility, and, doubtless, to each one specific effects 

 are due ; thus, it appears from the experiments of DAGUERRE and NEIPCE, that the 

 tithonic rays have the quality of changing the constitution of resinous bodies. There 

 is, indeed, scarcely one of the productions of vegetable life which is not characterized 

 by the facility with which it undergoes mutations under the influence of these princi- 

 ples; thus, wax, which is yellow, bleaches rapidly; chlorophyl, which is green, turns gray; 

 the volatile oils harden, as is often observed; the oil of oranges turns red; guiacum, from 

 a yellowish brown, turns green or blue ; the colouring principle of most petals is destroy- 

 ed; for example, the beautiful red of carthamus, and the purple of violets, is rapidly 

 bleached ; woody fibre also undergoes a disintegration, and, indeed, there is scarcely a 

 vegetable product on which some one of the various radiations does not leave a specific 

 impression. The extensive series of experiments made by Sir J. HERSCHEL on the 



