VITAL CHAEACTKRISTICS OP PLANTS. 397 



culation of fluids. This opinion is at least disputable, the sap of plants 

 ascending only once, — for that which is termed the descending sap of 

 the plant is the proper juice prepared in the leaf ; and the fact of 

 currents being observed in opposite directions, is no proof of the 

 existence of a circulation. But it may be asked, is the motion in the 

 Gha/ra or the cells of the Vallisneria spiralis, or in the hairs of the 

 radicle fibres oi frog's-bU, any proof of a circulation 1 It is certainly a 

 proof of the motion of a fluid in the cells of a plant, and is very dif- 

 ferent from a general circulation of the sap ; which is the only answer 

 that can be made to such an inquiry : and the true circulation in 

 animals is derived from an internal impelling power, and not from 

 external influences. 



A more distinctive character is obtained in the products of the 

 respiratory function in plants": respiration is performed by the entire 

 surface in most animals, as it is by all plants ; but the products are 

 diflFerent. In plants, the process consists chiefly in the conversion of 

 carbonic acid and water into vegetable matter ; hence oxygen is ex- 

 haled from the leaves, and carbonic acid absorbed by them from 

 the atmosphere ; and it is by the decomposition of that acid in the 

 leaf, that the greater part of the oxygen is restored to the air. And 

 although plants exhale carbonic acid during the night and in the shade, 

 yet the quantity is small ; and plants are, in reference to their respira- 

 tion, a balance in the opposite scale to animals ; they remove from the 

 air the carbonic acid exhaled from the lungs and spiracles of animals, 

 and re-supply the oxygen requisite for their respiration. Without the 

 vegetable tribes, the atmosphere would soon cease to be fitted for the 

 present race of animals ; without the carbonic acid formed by animal 

 respiration, plants would lose the greater part of their nutriment ; and 

 by their reciprocal action the atmosphere is preserved very nearly un- 

 changed. Therefore the most important difference between the two 

 may be said to be essentially that pointed out by Dr. Lankaster, in the 

 nature of the distinctive cha/racter of the gases inhaled and exhaled 

 by animals and by plants. With this brief survey, we may conclude 

 our comparative view of plants and animals by stating, that whilst they 

 are endued with many properties and functions common to both, they 

 possess others sufficiently distinctive, which prevent them from being 

 regarded as parts of the sarM link in the chain of vital existence. 



As we pass on to a more intimate examination of the various struc- 

 tures entering into a plant, it will be seen that we have objects of the 

 deepest interest presented to our notice ; and strikingly difiering as 

 we find plants and animals in some essentials, we shall here, at our 



