158 EESPIEATION OF PLANTS. 



presence of alkalies. The fixation of carbon and hydrogen from the 

 decomposition of carbonic acid and water gives rise to the formation 

 of the various secretions found in the bark and external cells, as chloro- 

 phyll, resin, oil, caoutchouc, and wax. 



Carbonic acid in solution, as has already been noticed, is taken up 

 in large quantity by the roots of plants from the soil, and it is also 

 absorbed from the atmosphere by the leaves. It may even be formed 

 in the cells of plants during the various chemical changes connected 

 with the elaboration of their juices and secretions. In the interior 

 of plants it is changed in various ways, but it is in the leaves more 

 especially that its decomposition takes place. At night it is given off 

 unchanged, by what Liebig considers as a mere process of exosmose, 

 in consequence of the dissolved acid being no longer assimilated by 

 the action of light. The quantity of this acid given off during the 

 night is by no means equal to that which is absorbed by the plant 

 during the day. 



The parts of plants which are not green seem to absorb oxygen. 

 Thus, roots and subterranean organs act in this way, and the presence 

 of oxygen seems to be necessary for their growth. There are also 

 certain periods in the life of a plant when carbonic acid is very largely 

 given off, even during the day, depending on a chemical change taking 

 .place in the starch of the plant, by which it is converted into sugar. 

 These periods are germination, flowering, and fruiting. The changes 

 alluded to wiU be discussed when these subjects are considered. 

 When plants are decaying, or are in an unhealthy state, they undergo 

 chemical changes, by which carbonic acid is formed. 



Aquatic plants have the power of decomposing carbonic acid 

 highly developed, and thus the preservation of the purity of lakes 

 and ponds is provided for. In Batavian ponds Pistia Stratiotes is 

 remarkable for its purifying effects, and Sir-H. Davy notices the great 

 vigour of aquatic plants in the lake Solfatara, where carbonic acid 

 was constantly bubbling up on the surface. The oxygenation of the 

 water by aquatics has also been observed by Morren of Geneva. 



In conclusion, three views of the respiratory process in plants have 

 been advanced — 



1. That oxygen is exhaled in large quantity during the day, and a 



moderate quantity of carbonic acid given off diuring the 

 night. 



2. That carbonic acid is exhaled in greater or less quantity at all 



times, but during the day it is decomposed, so that oxygen is 

 evolved. 



3. That no carbonic acid is evolved by leaves in a healthy state of 



the plant, but the elimination of oxygen only occurs. 



The last view is not now accepted by physiologists. Of the 

 others each has a number of adherents — many able physiologists 



