148 VEGETABLE PHYSIOLOGY. 



calcareous crust which often envelops it like a bark, numerous granules are seen 

 floating in a transparent liquid within the cell, and forming a current which rises 

 along one of the lateral walls, then flows horizontally along the upper wall, then 

 descends along the other lateral wall, and becomes again horizontal along the lower 

 wall of the cell. It is this intracellular motion which has been called rotation, a 

 very inappropriate term, for which it would be better to substitute that of cydosis 

 (abolished by Hugo Mohl), which expresses much more exactly the circular move- 

 ment of the sap in the cell. 



Respiration. The carbon of plants is derived from the carbonic acid contained 

 in the air ; the roots absorb it with the water of the soil which holds it in solution ; 

 whilst the carbon of the air enters the leaves through their stomata. Many 

 experiments prove that the leaves and green parts exclusively possess the power of 

 decomposing carbonic acid, thus separating the oxygen, and restoring it to the 

 atmosphere ; they also decompose water and retain the hydrogen ; this power is 

 only exercised under the influence of sunlight. Now animals are constantly burning 

 carbon by means of the oxygen of the air, and exhaling carbonic acid, in which 

 operation they consume an enormous quantity of oxygen ; but plants, by their 

 respiration, restore the balance, for they provide an inexhaustible store of pure 

 oxygen, and incessantly repair the loss which the atmosphere has sustained through 

 the respiration of animals. 



The power possessed by leaves of decomposing carbonic acid ceases at night or 

 in darkness ; then the carbonic acid, absorbed by the roots with the water of the 

 soil, enters the stem, and remains dissolved in the sap with which the plant is 

 impregnated ; soon this water evaporates through the leaves, and carries off the 

 carbonic acid which it held in solution. 



The green parts of plants absorb oxygen during the night by a chemical process, 

 which tends to produce a change in the materials contained in their tissues. To 

 blanch plants, they must be placed under the same conditions as the green parts of 

 vegetables are during the night, namely, in continuous darkness ; the carbonic acid 

 is then not assimilated, the green chlorophyll is not formed, and their tissues con- 

 tain an excess of water ; and the horticulturist is thus enabled to expel the bitter 

 principle from stems or leaves. 



This exclusive property of Ihe green parts is perhaps due to their having 

 absorbed the chemical rays of the solar light, which rays may aid in the decom- 

 position of carbonic acid in the chlorophyll. 



Respiration, which is the reciprocal action of the sap upon the air, and of the 

 air upon the sap, is carried on in the intercellular spaces (lacunae) beneath the 

 slomata, where the air comes into contact with the parenchyma. Submerged plants, 

 which have no epidermis, and whose parenchyma is hence exposed to the fluid, 

 decompose the carbonic acid which the water always contains, under the influence 

 of light transmitted through the water ; they fix the carbon and reject the oxygen, 

 which remains in solution, and supports the life of aquatic animals. Here, as in 

 the air, the Animal and Vegetable Kingdoms reciprocate only under the stimulus 

 of light; and if the water be too deep, the plant becomes pale and etiolated. 



