64 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



(as in the Cactus tribe) the leaves are wanting and the enlarged succu- 

 lent stem supplies their place. When these surfaces cease to become 

 green, the decomposing action also ceases ; carbon is no longer fixed 

 and oxygen set free ; but, on the contrary, carbonic acid is exhaled : 

 this is the case when the leaves change colour, previously to their fall, 

 in the autumn. The compounds which are thus generated in the green 

 surfaces, are conveyed to the remote parts of the fabric, by the circu- 

 lation of the sap, and become the materials of their nutrition ; and thus 

 the green cells of the leaves have exactly the same function, in minis- 

 tering to the growth of the fabric of the largest tree, which the green 

 cells of the humble Conferva perform in regard to themselves alone. 



84. It has been already mentioned, that the decay which is always 

 taking place in the softer vegetable structures, gives rise to a continual 

 production of carbonic acid, even in the living plant ; this process, which 

 must be regarded as a true Respiration, is effected, as in Animals, by 

 the union of the carbon of the Plant with oxygen derived from the atmo- 

 sphere ; and it is carried on, not by the green parts only, but also, per- 

 haps chiefly, by the darker surfaces. Being antagonized during the 

 day by the converse change just described, it can only be made sensible, 

 by placing the plants for a time in an atmosphere in which no carbonic 

 acid previously existed; and it will then be found that, even in full 

 daylight, a certain amount of that gas is exhaled. The fact, however, 

 becomes much more obvious at night, or in darkness ; since the decom- 

 position of the surrounding carbonic acid by the green surfaces is then 

 completely at a stand, and the full effect of the respiratory process is 

 seen. Moreover, when a plant becomes unhealthy, from too long con- 

 finement in a limited atmosphere, it begins to exhale more carbonic acid 

 than it decomposes ; and the same is the case, as just now stated, in 

 regard to leaves that have nearly reached the term of their lives. It 

 does not admit of question, however, that, under ordinary circumstances, 

 nearly the whole carbon of a slow-growing plant is derived from the car- 

 bonic acid of the atmosphere ; either directly through the leaves, or 

 indirectly by absorption through the roots ; and that there must be a 

 vast surplus, therefore, of the carbonic acid decomposed, over that which 

 is exhaled, during the whole life of the tree, that surplus being in fact 

 represented by the total amount of carbon contained in its tissues. 



85. It is probable that the minute amount of Carbonic Acid at present 

 contained in the atmosphere, is as much as could be beneficially supplied 

 to Plants, under the average amount of light to which they are sub- 

 jected, over the whole globe, and throughout the year. It has been 

 clearly shown, that, under the influence of strong sunlight, an atmo- 

 sphere containing as much as 7 or 8 per cent, of carbonic acid may be 

 not merely tolerated by Plants, but may be positively beneficial to them, 

 producing a great acceleration in their growth ; but as soon as the light 

 is withdrawn, it acts upon them most injuriously, causing them speedily 

 to become unhealthy, and .altogether destroying their vitality, if they 

 are long subjected to it. Under more cloudless skies than ours, how- 

 ever, the continual supply of a larger quantity of carbonic acid, than 

 our atmosphere contains, is found to be quite compatible with healthy 

 vegetation ; especially in the case of Cryptogamic plants, which (as will 



