232 LECTURE XII. 



from without. Mayer is of opinion that the oxygen absorbed whilst the 

 plant is in darkness is retained in the form of an organic acid, and that 

 it is in consequence of the reduction of this acid (he believes it to be 

 an isomer of malic acid) that oxygen is evolved and starch is formed 

 in the chlorophyll- corpuscles when the plant is exposed to light. He 

 conceives the mode of this decomposition to be the following. Relying 

 upon the fact that when uranium succinate is exposed to light it is 

 decomposed into uranium propionate and carbonate, he infers that when 

 this malic acid is exposed to light in a cell containing chlorophyll, lactic 

 acid and carbon dioxide are formed, and he points out that sugar may 

 be produced by polymerisation from the lactic acid. He actually found 

 that a mixture of malic acid and calcium malate was decomposed when 

 exposed to sunlight ; carbon dioxide was abundantly evolved, but neither 

 lactic acid nor sugar could be detected in the liquid: it appears that, 

 under the conditions of the experiment, the lactic acid underwent further 

 decomposition. The evolution of oxygen by the plant under the in- 

 fluence of light is doubtless due to the decomposition of the carbon 

 dioxide thus formed, and with this the appearance of starch-grains in 

 the chlorophyll-corpuscles is to be connected. 



In addition to their possible significance in the construc- 

 tive metabolism of plants, the organic acids are of use in 

 other ways. We have seen, for instance (p. 41), that it is 

 probably to the presence of them that the turgidity is to be 

 ascribed ; that the presence of acid sap in the root-hairs 

 (p. 55) renders possible the solution and absorption of 

 mineral substances which are insoluble in water ; that oxalic 

 acid, at least, decomposes the salts absorbed by the roots 

 (p. 149). 



We will now pass on to the consideration of the origin 

 of the aromatic substances. We have learned already (p. 21) 

 that an aromatic substance (lignin) may be formed from a 

 carbohydrate (cellulose), and Hoppe-Seyler's observations 

 (p. 228) suggest a mode in which this may take place. 

 Hartig observed, and his observations have been confirmed 

 and extended by Wiesner, that the parenchymatous cells 

 pf the trunks of many trees (Beech, Elm, Sycamore, Oak) 

 contain grains of resin. They conclude that these resin-grains 

 are derived from starch-grains, and it appears very probable 

 that tannin, an aromatic glucoside, is formed as an ante- 

 cedent of the resin. We would have then, in the process of 



