THE DIGESTION OF PLANTS. 21 



motion, and go in search of his food ; a plant, a reducing apparatus, has a more delicate 

 duty to perform, and Nature herself becomes its minister, and offers it a supply for all 

 its wants. 



55. Under these circumstances, therefore, when leaves of aerial plants are placed in 

 carbonated water in sunshine, we can, so long as their structure remains unimpaired, ob- 

 serve with a certain degree of correctness the phenomena which they would exhibit 

 under more natural conditions. By botanists and the earlier writers on chemistry, the 

 function discharged by the green parts of plants is often spoken of as a species of res- 

 piration, analogous, to a certain extent, to animal respiration. This mistake originated 

 in those obscure physiological views of which the old doctrine of vitality was the pro- 

 lific parent. Respiration is essentially an oxydizing process, a process of combustion, 

 but the part which is played by a vegetable leaf is to obtain carbonaceous matter from 

 air, and store it up in a solid form in its various strictures. This solid matter thus ob- 

 tained is the very same which at an after period, entering the digestive organs of ani- 

 mals, is by them subjected to assimilatory processes, and finally becomes a part of their 

 fabric. The action which is performed by those green parts is not, then, a respiratory 

 action, but one of digestion. 



56. We have already said that leaves placed in spring water in sunshine evolve bub- 

 bles of gas. A direct sunshine is not, however, absolutely required; the same experi- 

 ment, if conducted with proper precautions, can be made to succeed with the diffused 

 skylight. The bubbles, as they form, rise to the top of the water, and if collected and 

 analyzed, yield, as in the case of water-plants, two substances, oxygen and nitrogen; a 

 little carbonic acid is always present, but that arises from the peculiar conditions under 

 which the experiment is made (Ap., 794). 



57. Thus, by the influence of the sunlight, organic matter is added to vegetable sys- 

 tems, the action being accompanied by a variety of chemical decompositions and interstitial 

 diffusions. The substances arising are such as are necessary for the uses of the plant, 

 and in order to distribute them to the requisite parts, mechanical motion has to take 

 place. This, in the more highly organized plants, goes under the designation of the 

 flow of the sap. The descending sap, like the arterial blood of animals, contains all 

 the different compounds which are required by the organized structure. We shall in 

 the next chapter consider the causes which direct the movements of this liquid. 



