VEGETABLE RESPIRATION. 83 
the absence of heat, and the whole plant must be in a condi- 
tion differing considerably from its state during the day. 
282. From what has been said, it will be seen that we are 
still very much in the dark regarding the nature and uses of 
the action which takes place in the leaf, and the influence of 
external agents upon it. It is not unreasonable to suppose 
that the leaf is not regulated in its action entirely by the de- 
gree of light present. It must be borne in mind that a plant 
‘is a being endowed with life, and capable, to a certain ex- 
tent, of adapting the exercise of its functions to the circum- 
stances in which it may be placed; and we may fairly pre- 
sume, that the amount of carbon taken in by the leaf, or 
given out by this organ, will vary much according to the 
quantity in the sap derived from the soil, which, it is evi- 
dent, must be very different in different situations. 
283. Plants that are not green do not produce the same 
changes upon the atmosphere as those of this colour. Mush- 
rooms, for example, produce carbonic acid, deteriorating 
the air, and they produce this effect during the day as well 
as during night. 
284. The sap, having in the leaf been rendered fit for the 
nutrition of the plant, descends in the innermost layer of 
the bark, being here called proper juice, assists in preparing 
the cambium, which acts such an important part in the for- 
mation of the new layers of wood and bark, and spreading 
horizontally through the medullary rays, produces the dif- 
ferent secretions which are found in the vegetable. Of these 
secretions, it is most probable that some are destined to renew 
the cambium along with the descending sap of the ensuing 
season, and someto qualify the ascending sap to promote nis 
evolution and growth of the ~~ 
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