220 BOTANY PART i 



oxygen, containing, however, under such simple conditions of experi- 

 ment, traces of other gases derived by diffusion from the plant and 

 the water. As water absorbs much less oxygen than carbonic acid (at 

 a temperature of 14 C. 100 vols. of water will dissolve only 3 vols. 

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

 oxygen become visible; whereas the flow of the carbonic ;icid dis- 

 solved 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 assiniilatory activity. Similarly an 

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

 According to KREUSLER assimilation in sunshine attains its maximum in air con- 

 taining about 10 per cent of carbonic acid ; with a higher percentage it begins to 

 decrease. If the amount of carbonic acid gas be increased three hundred times 

 (from 0*03 per cent to 10 per cent in the atmosphere), the formation of starch is 

 only increased 4-8 times, while an increase of the C0 2 to six times the normal 

 proportion results, according to H. BROWN, in the formation of six times as much 

 starch. A longer exposure to an atmosphere rich in CO., does not have a favourable 

 influence on the health of the plant ; it takes on abnormal modes of growth, loses 

 the power of flowering, and thus makes it evident that it is suited to live in 

 air with the ordinary proportion of C0 2 . This may be termed the "harmonic" 

 optimum ( 40 ). 



Like other vital activities of the plant assimilation is dependent upon the 

 temperature, and at lower grades of temperature is not so great as the available 

 light and carbon dioxide would permit. In bright, warm weather, on the other 

 hand, the small amount of C0 2 in the atmosphere becomes the limiting factor. 



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

 place of the carbon dioxide, and is poisonous to plants, though less so than to 

 animals. 



Under the same external conditions, the assiniilatory 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 carbohydrate. 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 

 structure of the leaves, but, without doubt, has also its cause in the greater or less 

 intensity of the assimilatory process itself. 



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, according to SACHS, form in 

 a summer day of fifteen hours about 25 grammes starch per square 

 metre, while Cafalpa, according to BROWN and ESCOMBE, forms about 

 3 grammes per hour and square metre of surface. (The carbon for 

 the formation of the starch was supplied in the first cited example 

 from 50 cubic metres of air. A room of 120 cubic metres would 

 accordingly contain enough carbonic acid for 60 grammes of starch.) 

 From these figures a faint conception may be gained of the enormous 

 activity of the assimilatory processes, of the vegetation of the surface 

 of the earth. The German wheat harvest for 1900 represented about 

 23,000 million kilos of assimilated substance. 



