188 EXTRICATION OF CARBONIC ACID BY RESPIRATION. 



fluid, and acquires that succulence or juiciness, which is so much 

 valued in such vegetables. 



289. But though the bulk of plants which are undergoing 

 this treatment, may considerably increase, yet the weight of their 

 solid contents diminishes ; for during the whole period, respiration 

 is going on; and, as there is thus a constant loss of carbon, whilst 

 little or none is introduced, it follows that, if the tissues were 

 dried by heat, they would shrink to less than their former amount. 

 This is found to be the case ; and it is also true of a seed in the 

 process of sprouting or germinating, which constantly diminishes 

 in the weight of its solid contents, up to the time when some of 

 the new-formed leafy surfaces become green, and begin to absorb 

 carbon from the atmosphere. 



290. At this period of the growth of the young plant, it may 

 be regarded as having a curious analogy with the tribe of Fungi. 

 Both are supplied with nutriment already organised ; for whilst 

 the one has it previously stored up by its parent, the other receives 

 it from the decomposing matter, upon which alone it can grow. 

 Both are developed most rapidly and luxuriantly in the absence 

 of light, if well supplied at the same time with warmth and 

 moisture. And the Fungi, like the germinating seed, give out 

 a large quantity of carbonic acid to the atmosphere, without 

 receiving any carbon from it ; since the peculiar character of the 

 aliment they imbibe by their roots, renders any additional sup- 

 ply of this ingredient unnecessary. In the Fungi, therefore, we 

 have the process of respiration as distinct and easily under- 

 stood, as in Animals, both classes of being subsisting upon 

 food already organised, in which there is a large proportion of 

 carbon. 



291. The process of digestion is confined, as before stated, to 

 the leaves, and to those green surfaces of plants, which corre- 

 spond with them in function ; but that of respiration^ although 

 performed by the leaves more energetically than by any other 

 part (at least during the ordinary process of growth), is not 

 restricted to them, but is partially effected by the other surfaces, 

 and even by the roots. The knowledge of this fact is important ; 

 since, through ignorance of it, much valuable timber has been 



