Comparative Physiology. 345 



of the carbon thus furnished ; the formation of nutritious pro- 

 ducts, and the elaboration of peculiar secretions, are so com- 

 pletely subservient to it, that they languish under a diminution, 

 and usually cease under a continued abstraction, of its agency. 

 The degree of light necessary to plants is very various. Fungi 

 are found in caverns and mines; and Humboldt met v^itli both 

 endogenous and exogenous species presenting a green colour, in 

 the subterranean galleries of the Freyberg mines. Plants grow 

 towards the light : the roots, however, avoid the light ; and many 

 of the simpler plants also, as mosses, ferns, &c., which grow on the 

 north and north-west sides of trees and rocks, while the opposite 

 parts are comparatively bare. By throwing the light from 

 below upwards, by a mirror, among seeds placed in moss, and 

 darkening them from other light, M. Schultz found the order 

 of developement reversed, the roots sent upwards and the stem 

 downwards, showing thus the influence of light on the direction 

 of the stem and roots. In germination the influence of light 

 rather retards than hastens vegetation, the tendency of light to 

 decompose and fix carbon being opposed to the chemical action 

 then required." 



On the developement of light in plants, " it has been asserted 

 that many flowers, as nasturtium, marigold, sunflower, &c., 

 disengage light in warm summer evenings, but it is doubted. 

 There is no doubt, however, that light is emitted by many 

 fungi, especially various species of Rhizomorpha, and, in some 

 instances, to a very considerable extent. The light is perceived 

 in all parts of the plant, but chiefly in the young Avhite shoots ; 

 and it is more vivid in young than in old plants. The phospho- 

 rescence is stronger in such as grow in the moist and warm 

 localities of mines, than in those inhabiting dry cold situations. 

 It ceases if the atmosphere is deprived of oxygen, and reappears 

 when restored to the air. The juice of the jE'uphorbm phosphorea 

 emits light, especially when heated. These evolutions of light 

 seem connected with the combination of carbon and oxygen ; it 

 takes place also from dead and decaying wood, but is not in- 

 creased when the substance is placed in pure oxygen. In 

 animals, where it is more common, it appears to be occasioned 

 by the secretion of a product possessing a luminous j^roperty, 

 depending for its continuance on the life and health of the 

 animal." 



The power of light has been thought to lie principally in its 

 chemical action, though in the sunbeam it is always accompanied 

 with heat and electricity, and it must be difficult to separate the 

 action of each. A white light is always indicative of intense heat, 

 the latter generally producing the former, Avhen raised to a high 

 pitch; they so generally accompany one another, the one always 

 increasing the action of the other, that it is difficult to talk of them 

 separately. Heat and light are like electricity and magnetism, 



