54: Causes of Ignorance hi ArI)oricuUu?'e, 



the leaves, is there exposed to the action of the atmospheric 

 air, much in the same way as tlie blood in lungs; with this 

 hnportant difference, that, while animal blood principally 



absorbs oxygen, and gives out carbonic acid gas, &c 



sap appears to have the greatest affinity for carbonic acid, 

 and to be compelled to give out a portion of its oxygen, 

 before it is fit to afford proper nourishment to the plant. 

 As the sap, during the process of its elaboration in the 

 leaves, has not only to imbibe certain qualities, but to throw 

 off others, plants, like animals, are provided with the neces- 

 sary organs for a double kind of respiration [which Mr. 

 Howden would rather have called excrements. Have excre- 

 ments organs?]. These functions, however, are never per- 

 fectly performed without the assistance of light. When a 

 plant is exposed to the full action of the sun's rays, it gives 

 out pure oxygen, and absorbs carbonic acid ; which is after- 

 wards decomposed in the plant, the carbon being only re- 

 tained. Carbon has been proved to be essential to the 

 existence of plants ; and it appears that their vigour depends 

 upon the quantity of carbonic acid they are enabled to ab- 

 sorb.'* 



Lecture 6. (p. 621.) : — " He had before stated that leaves 

 absorb carbonic acid during the day, and oxygen during the 

 night, forming carbonic acid in proportion to the oxygen 

 they have absorbed ; that they decompose their carbonic acid 

 during the day, setting free the greatest part of the oxygen it 

 contains, and retaining the carbon, which appears to afford 

 them food ; and that this process is necessary to their vege- 

 tation, though the exact manner in which it acts is at present 

 unknown to botanists." 



Thus far Mr. Lindley, on the use of the leaves to imbibe 

 nourishment or food for the support of trees ; consequently, 

 to assist in the growth of timber. Before I proceed to the 

 other question, Whether the branches and leaves are the 

 principal means of the increase of timber, or are perfectly 

 useless to this end, I would just beg leave to ask Mr. How- 

 den, and others that are" of his opinion (as I have asked 

 before in my publications), whether trees that grow in hori- 

 zontal fissures of sandstone and other rocks, with immense 

 thicknesses or layers of rock over and below their roots, and, 

 of course, no soil, and where no rain or dew can get at their 

 roots, and, in some countries, where it seldom if ever rains ; 

 I ask, whether such rocks do contain all the necessary ingre- 

 dients or matter that constitute the food of trees ; and that the 

 roots actually imbibe, extract, or introsuscept (as Sir Henry 

 Steuart would say), all the sap or blood necessary for the 



