178 THE MICKOSCOPE. 



cending only once, for that which is termed the descend- 

 ing 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 Chara or the 

 cells of the Vallisneria spiralis, or in the hairs of the 

 radicle fibres of frog's-bit, any proof of a circulation ? It 

 is certainly a proof of the motion of a fluid in the cells of 

 a plant, and is very different 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 per- 

 formed by the entire surface in most animals, as it is by 

 all plants ; but the products are different. In plants, the 

 process consists chiefly in the conversion of carbonic acid 

 and water into vegetable matter ; hence oxygen is exhaled 

 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 respiration, 

 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 atmo- 

 sphere 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 unchanged. Therefore the most 

 important difference between the two may be said to be 

 essentially that pointed out by Dr. Lankester, in the nature 

 of the distinctive character of the gases inhaled and exhaled 

 "by animals and by plants. 



Dr. Carpenter accepts this as a sufficiently distinctive 

 line of demarcation between the two kingdoms. In an 

 address to the Microscopical Society, he says : " I wish to 



