198 Comparative Physiology. 



apparatus of plants being required by the locomotive propen- 

 sities of animals, and the nature of their food. Nerves and 

 muscles are needed for the motions of the stomach ; and for the 

 regularity and constancy of the movements of the blood in the 

 circulating apparatus, the peculiar contractile powers of mus- 

 cular fibre are needed in the impelling organ, the heart." 



The distinction here drawn between plants and animals, in 

 the latter for the most part possessing a digestive cavity, and a 

 special circulating apparatus, points out the soil as the stomach 

 of the plant, and the diffused contractility of the vegetable 

 system, united to the power of endosmose, as the circulating 

 power. As in the stomach and alimentary canal of the animal 

 the food is prepared for absorption, so in the soil, by the action 

 of heat and moisture, and of the air and electricity, the food of 

 the plant is reduced to a soluble state, fit for solution in water, 

 without which it cannot be absorbed by the spongioles of the 

 root, which have been thought, with apparent reason, to have 

 also an action of their own on the food. We all know the 

 necessity of keeping the soil in an open pulverised state, to 

 admit air, heat, and moisture freely, retaining as much as pos- 

 sible of the former, and only as much of the water as can be 

 held in absorption by the particles of earth themselves. The 

 circulation, though of a much feebler kind than that of animals, 

 is carried on by the process of endosmose, or the power of 

 denser fluids, inside of a membranous sac, attracting the thinner 

 fluids outside, by which means the sap is raised to the leaves, 

 where it is elaborated and evaporated to a more dense consist- 

 ency, and attracts the more fluid ascending sap. This power, 

 united to the diffused contractile power of the vessels, supposed 

 to be caused by electricity, assisted, perhaps, by gravity in the 

 descending sap, produces the circulation, all of which will be 

 found treated more particularly when we come to absorption and 

 circulation. The respiration is carried on through the whole 

 surface, but principally in the leaves. Some have thought the 

 action in the leaves similar to digestion ; but though carbonic 

 acid is absorbed, and water evaporated, which causes it to differ 

 from animal respiration, and though the chemical power of 

 transformation in the sap is no doubt assisted by light, yet 

 similar transformations take place in the blood of animals, where 

 light does not act, and perhaps too much is ascribed to the 

 power of light on the leaves. 



The power of light is, perhaps, as much stimulant as chemical, 

 as light itself without the leaves will not act as it does with their 

 assistance. Aeration in leaves, though different from that in 

 lungs, is likely a similar function. At all events, in com- 

 parative physiology, leaves can only be viewed as the respiratory 

 apparatus, if we wish to classify organs for the purpose of com- 



