BY JAMES M. PETJRIE. 541 



These, however, cannot be detected in the living plant, since the 

 aldehyde is instantly polymerised to carbohydrate by the proto- 

 plasm, and the peroxide is catalysed by an enzyme, with evolution 

 of oxygen. Leaves in which the protoplasm and enzyme have 

 both been killed, when placed under conditions favourable to 

 assimilation, develop formic aldehyde and hydrogen peroxide, and 

 these accumulate until the latter has destroyed and bleached all 

 the chlorophyll, and the photolytic process is stopped. 



In applying this photosynthesis of formaldehyde to the case of 

 the Nettle-Tree, it seems not improbable that under certain 

 altered conditions (as in the absence of the enzyme) the aldehyde 

 may be oxidised by the hydrogen peroxide to formic acid : — 

 HCHO + H 2 0„ - HCOOH + H 2 0. It is certain, however, that 

 acetic acid is never formed in this way.- We are then led of 

 necessity to the unlikely view that the two acids, so closely allied 

 chemically and so intimately associated in the metabolism of the 

 Nettle-Tree, are formed by entirely different processes in the 

 plant. 



Liberation of Acids from their Salts.— The fresh leaves were 

 found to contain 80 % of water. This gives us in every 100 parts 

 of sap, 224 of formic and acetic acids, and 694 of their salts, 

 and is approximately a 1 % solution in the plant. 



Now every organism, both plant and animal, by means of its 

 self-regulating function, strives to prevent any variation in the 

 ratio of its hydrogen and hydroxyl ionic concentrations to which 

 its cells are habituated, by forming acid or alkali as required. 

 We must therefore regard the formation of acid as being able to 

 excite and call up an equivalent amount of base in the mineral 

 solution from the soil, until a certain equilibrium is obtained. It 

 is not necessary that this equilibrium point indicate neutrality, 

 it is simply the balance of a number of reversible reactions of 

 production and dissociation. 



In the first place, by the principle of mass action, carbonic 

 acid can replace a little acetic acid from its salts : — 

 CaA, + CO, + H 2 = CaCO :J + 2HA. 

 This may occur with the assimilation in the leaves, and there is 



