338 Comparative Physiology. 



The proportion of light is much greater in the polar regions 

 than the proportional vigour of vegetation there ; and heat, 

 generally accompanied by electrical changes, would appear most 

 powerful. Heat appears to act much by its expansive power, 

 as may be seen in the greater elongation between the joints of 

 plants exposed to its influence. It is also indispensable to the 

 chemical and vital powers of plants ; but, unless the chemical 

 power is increased by the presence of light in proportion to 

 heat, a weak, elongated, feeble growth is produced. The motion 

 of the air is also necessary to invigorate the growth ; and there 

 are probably also other substances got from the air besides car- 

 bonic acid, as we shall see when we come to the article secretion. 

 On all these accounts, an influx of air, in a moderate degree, is 

 found beneficial and indispensable in all forcing-houses, when 

 the heat can be kept up during its admission ; the vegetation of 

 seeds, and subsequent growth below bell-glasses in heat, are rapid 

 indeed, but proportionally weak. The heat sets in motion the 

 sap, by the evaporation setting the endosmose power to work, 

 and stimulating the excitability and vital properties of the tis- 

 sue ; and heat and moisture, with the nutriment contained in 

 the water itself, and the food it carries in solution, are indis- 

 pensable to all vital action in plants. The heat also assists the 

 chemical action of the light and of the organs, as we find in 

 wall fruits, in the juicy acid kinds of which there is always more 

 sugar on walls ; while, on the other hand, some sweet dry fruits 

 are hurt by the increased chemical action producing more starch, 

 and cavising dry mealy fruit on walls, when the same sort on 

 standards would have sweeter and more juicy fruit. Fruit will 

 ripen in heat without light, as in those preserved in warm 

 places, where there is not much light, and mixed among sand, 

 oatmeal, &c., where light is totally excluded. In the bulbs 

 formed below ground, as before noticed, heat also acts independ- 

 ent of light. The heat and electricity of the sunbeam are also 

 useful in increasing the power of the leaf to decompose carbonic 

 acid. Light will not decompose carbonic acid in the same way 

 out of the leaf, as it does by means of the leaf; and the power 

 is therefore to a great degree organic. It is the prevalent 

 opinion that the chemical power of light is the principal agent. 

 Dumas describes it as stored up in the leaf for that purpose : 

 but it may still be doubted, as Schleiden and others seem to 

 suppose, whether the heat and electricity of the sunbeam are 

 not important agents in increasing the action of the leaf, which 

 they think more organical than chemical. Organical action and 

 chemical, heat, light, and electricity, are so intimately connected 

 in their causes and effects, that it is difficult to separate and 

 define. 



Heat injures all seeds by drying ; though, after consider- 

 able exposure to a dry heat, no physical alteration is per- 



