100 Mr. Burnett on the Development 



formed chiefly by the leaves and petals, though also in a less 

 degree by the stems and roots, like the respiration of animals, 

 is attended with and marked by the conversion of oxygen into 

 carbonic acid : it is the respiration of plants. 



Again, vegetables at certain times and under certain cir- 

 cumstances decompose carbonic acid, and renovate the atmos- 

 phere by the restoration of its oxygen; but this occasional 

 restoration is dependent not on the respiratory, but the diges- 

 tive system ; it in part arises from the decomposition of water, 

 but chiefly from the decomposition of carbonic acid, absorbed 

 either in the form of gas or in combination with water, either 

 by the roots or leaves, or both: and here, again, the analogy 

 holds good between the functions of respiration and digestion 

 in animals and plants, for to both is carbonic acid deleterious 

 when breathed, and to both is it invigorating to the digestive 

 system when absorbed as food. 



The presence or absence of light seems to have little or no 

 influence on the respiration of vegetables ; but it produces very 

 notable effects on their assimilating powers, by enabling the 

 specific vitality of the plant to separate from air and water 

 those principles which they hold in solution or combination, 

 which are necessary for their support, and to liberate such 

 others as may be too abundant in the crude aliment they intro- 

 suscept ; for plants growing in the dark become etiolated, and 

 assimilate but little solid matter, and scarcely ever form their 

 appropriate and peculiar secretions : thus assimilation tends to 

 increase, respiration to decrease the solid materials of the plant. 

 Another agent of great power, though hitherto scarcely noticed, 

 is the electricity generated by plants, and excited by their pro- 

 fuse perspiration. I have frequently found, by experiments 

 such as have been related, the perspiration of leaves and plants 

 to be a moiety of their own weight, and this for days and 

 weeks together: and in some extreme cases, as was shewn 

 by Hales, plants will perspire during twenty-four hours as 

 much water as is equal in weight to two-thirds of their gross 

 bulk. This abundant exhalation must be designed to answer 

 some important end, and this use I believe to be (as at another 

 time I shall attempt to shew) to keep the plant constantly in 

 that electrical state which will favour the entrance of fluid into 



