sect, ii PHYSIOLOGY 201 



vols, of oxygen, but 100 vols, of carbonic acid), the escaping bubbles 

 of oxygen become visible ; whereas the flow of the carbonic acid 

 dissolved in the water to the assimilating plant is imperceptible. 



Artificially conducting carbonic acid through the water increases, to a certain 

 degree, the evolution of oxygen, and thus the assimilatory activity. Similarly an 

 artificial increase of carbonic acid in the air is followed by increased assimilation. 

 In sunshine assimilation attains its maximum in air containing about 8 per cent of 

 carbonic acid ; with a higher percentage it begins to decrease. If the amount of 

 carbonic acid gas be increased two hundred times (from 0'04 per cent to 8 per cent in 

 the atmosphere), the formation of starch is only increased 4-5 times. 



Carbon monoxide (CO) cannot be utilised by green plants ; it cannot take the 

 place of the carbon dioxide, and is poisonous to plants. 



Under the same external conditions, the assimilatory activity of different 

 plants may vary from internal causes. In the same time and with an equal leaf 

 surface, one plant will form more, and another less carbohydrates. In this sense, 

 it is customary to speak of a "specific energy of assimilation,'' which is partly due 

 to the different number and size of the chloroplasts, as well as to a difference in the 

 air-spaces and consequent aeration of the leaves, but, without doubt, has also its 

 cause in their greater or less energy. 



As examples of medium assimilatory activity, the leaves of the 

 Sunflower and Pumpkin may be cited. Under conditions favourable 

 for assimilation, the leaves of these plants form in a summer day of 

 fifteen hours about 25 grams starch per square metre. The carbon 

 for the formation of the starch was supplied in this case from 50 

 cubic metres of air. A room of 120 cubic metres would accordingly 

 contain enough carbonic acid for 60 grams of starch. From these 

 figures a faint conception may be gained of the enormous activity of 

 the assimilatory processes, which are necessary to furnish the yearly 

 grain supply of a large country. 



The Utilisation of the Products of Assimilation 



The Formation of Albuminous Substances.- — The chlorophyll 

 bodies supply plants with organic nourishment in the form of a 

 carbohydrate. Although the greater part of the organic plant sub- 

 stance consists only of carbohydrates, as, for example, the whole 

 framework of cell walls, yet the living, and consequently the most 

 important component of the plant-body, the protoplasm, is composed 

 of albuminous substances. These albuminous substances have a com- 

 position altogether different from that of the carbohydrates. In 

 addition to carbon, oxygen, and hydrogen, they also contain nitrogen, 

 sulphur, and frequently phosphorus, the nitrogen indeed in consider- 

 able proportion (about 15 per cent). There takes place accord- 

 ingly WITHIN PLANTS A NEW "FORMATION OF ALBUMINOUS SUBSTANCES 



FROM THE carbohydrates. There are certain indications that this 

 formation is, in part, accomplished within the green cells of the leaves, 



