138 PRODUCTIO* CF OXALIC ACID IN PLANTS. 



ready been detailed. It is found in small quantities in many plants. 

 The potash in forest trees is supposed to be in combination with oxalic 

 acid, while in the lichens oxalate of lime serves a purpose similar to that 

 performed by the woody fibre of the more perfect plant; it forms the 

 skeleton by which the vegetable structure is supported, and through 

 which its vascular system is diffused. 



The production of this acid in the living plant is readily understood 

 when its chemical constitution (Co O3) is compared with that of car- 

 bonic acid (COa). For 



2 of Carbonic Acid = C2 O4 



Difference . . . O^ 



That is to say, 2 of carbonic acid are transformed into 1 of oxalic acid 

 by the loss of 1 equivalent of oxygen — or generally, carbonic acid by the 

 loss of one-fourth of its oxygen may be converted into oxalic acid. 



But the leaf absorbs carbonic acid and gives off^ oxygen. In the lichens, 

 therefore, which contain so much oxalic acid, a large portion of the car- 

 bonic acid absorbed is, by the action of light, deprived of only one-fourth 

 of its oxygen, and is thus changed into oxalic acid. The same is true to 

 a smaller extent of the sorrel leaves and stems, which owe their sour- 

 ness to the presence of oxalic acid — of the leaves and stems of rhubarb 

 also — in a still smaller degree of the beech and other large trees, in 

 which much potash, and probably also of marine plants, in which 

 much soda is found to exist. It must be owing to the peculiar structure 

 of the leaves of each genus or natural order of plants, that the same ac- 

 tion of the same light decomposes the carbonic acid in different degrees 

 — evolving in some a less proportion of its oxygen, and causing in such 

 plants the formation of a. larger quantity of oxalic acid. 



The fact of the production of this oxalic acid, to a very considerable 

 amount in many plants, is a further proof of the uncertainty of those 

 experiments from which physiologists have concluded that the leaves 

 of plants emit a bulk of oxygen sensibly equal to that of the carbonic 

 acid absorbed.* 



I have referred the production of more or less oxalic acid in different 

 plants to the special structure of each, and this must be true, where, in 

 the same circumstances, different results of this kind are observed to 

 take place — as where sorrels and sweet clovers grow side by side. Yet 

 the influence of light of different degrees of intensity on the same plant, 

 is beautifully shown by the leaves of the Sempcrvivum arboreum, of the 

 Portulacaria afra, and other plants which are sour in the mornings tasteless 



* Were we permitted, in the absence of decisive experiments, to state as true what theo- 

 retical considerations plainly indicate, we should say — 



1°. That plants containing much oxalic or other similar acids, and not deriving much car- 

 bonic acid from the soil, must give off from their leaves a bulk of oxygen less than that of the 

 carbonic acid absorbed. 



2°. That plants containing no sensible quantity of such acids, nor fed by carbonic acid 

 from the soil, may evolve oxygen sensibly equal in bulk to the carbonic acid absorbed. 



3°. That if little of these acids be present, and mucli carbonic acid be absorbed from the 

 soil, the volume of oxygen given off by the green parts of the plant must be sensibly greater 

 than that of the carbonic acid they absorb. 



4°. Tli^the leaves of the pines and other trees containing much turpentine — in which 

 hydrogerWs in excess— must at all times give off oxygen in greater bulk than 'iie carbonic 

 acid they absorb. They must decompose water as well as carbonic acid, anl evolve the 

 oxygen of both. 



