Peteie. — On Vegetable Physiology. 427 



Aet. XXXVIII. — A Modern Chapter in Vegetable Physiology. 

 By D. Petrie, M.A., F.L.S. 



[Presidential Address to the Auckland Institute, 8th June, 1S96.] 



It is now rather more than a hundred years since the founda- 

 tions of an accurate knowledge of vegetable growth and nutri- 

 tion were laid. The first important result to be established 

 was that plants derive the whole of their carbon from the 

 carbon-dioxide of the air. And carbon, we must remember, 

 is the most important constituent of plant tissues, forming, as 

 it does, something like half the dry weight of neai'ly all plants. 

 Not long after this result was established came the proof that 

 the carbon-dioxide of the air is not directly assimilated by 

 plants, but undergoes decomposition in the chlorophyll-bear- 

 ing cells of the leaves under the influence of sunlight, the 

 carbon going to form part of the food of the plant, while the 

 oxygen combined with it is given back into the air. In course 

 of time it was seen that in the process of fixing this carbon 

 and uniting it with oxygen and hydrogen into carbo-hydrates 

 which finally appear as starch or sugar, large quantities of 

 energy are locked up in the products of the synthesis. The 

 energy thus condensed is wholly derived from the sun's rays ; 

 and it is now recognised as the sole source and origin of the 

 activity that animals of every grade display, as well as of the 

 energy locked up in coal, wood, and vegetable oils, which we 

 utilise for doing work in our steam-engines, and for giving 

 heat and light in our factories and homes. The green plant 

 thus appears in the guise of a living storage engine, focussing 

 and accumulating, for the use of man and of the animal crea- 

 tion in general, the energy which the sun unceasingly pours 

 upon our planet. The question of the origin of the carbon 

 contained in plant tissues was thus the first to be clearly and 

 permanently settled. The settlement showed that man can 

 do nothing to hasten or retard the process of carbon assimila- 

 tion ; and, further, that the stores of carbon-dioxide spread 

 through the air are more than sufficient to meet the daily and 

 hourly wants of every green thing that grows on the face of 

 the earth. Nature has spontaneously made the most ample 

 provision for the nutrition of plants so far as carbon is con- 

 cerned. 



While the process of carbon fixation was under discussion 

 speculation began to busy itself with the sources of the nitro- 

 gen contained in vegetable tissues. As plants derived their 

 carbon entirelv from the air, of which it forms but a minute 



