324 



THE FOOD OF PLANTS 



accumulate in sufficient amount to reach the limit inhibitory to further 

 assimilation of carbon dioxide '. 



Kreusler estimated that, allowing for the respiratory activity, the 

 leaves of Rnbns fruticosns in an atmosphere containing 0-3 per cent, of 

 carbonic acid gas decomposed in one hour per square metre of leaf-surface 

 2-5 grammes of carbon dioxide (= 1-54 grm. starch) when exposed to an 

 intensity of electric light corresponding to diffuse daylight. Such deter- 

 minations afford a more accurate measure of the intensity of carbon dioxide 

 assimilation than do estimations of the increase in the dry weight of the 

 leaf, for the loss by translocation can only be calculated with approximate 

 accuracy. 



Sachs, however, made various estimations in the latter manner by halving 

 a leaf, exposing one-half only to light, and then finding the difference in dry 

 weight between the two. It was thus found that the amount assimilated in one 

 hour per square metre of leaf-surface was, for Helianthus annuus 1-8 gramme, 

 and for Cucurbita pepo 1-5 gramme. Hence a vigorous sunflower with 145 leaves 

 (=1-5 sq. metre of leaf-surface) would obtain 36 grammes of organic material in 

 fifteen hours of daylight, while a plant of Cucurbita, with 1 16 leaves totalling 7-3 sq. 

 metre surface area, would gain 185 grammes. This productive activity is necessary 

 in order to provide the material accumulated by a sunflower in the course of 

 a single season, for this may amount to 2 kilos. In trees, owing to the enormous 

 leaf-surface, the total amount assimilated must obviously be much greater, for 

 a small tree of Aesculus hippocastanum, 8-5 metres high, possessed about 8,000 

 leaves with a total of 320 sq. metres of leaf-surface. 



It is immaterial for metabolic purposes whence the organic food is 

 derived, but, nevertheless, it does not follow that a leaf will be nourished 

 as well by food conveyed from other leaves as by that which it produces itself, 

 for the stoppage of the normal activity creates abnormal conditions which 

 may act injuriously upon the leaf and ultimately cause its death. Thus 

 leaves exposed to light in an atmosphere devoid of carbon dioxide fall ill 

 after a time, lose the power of photosynthetic assimilation, and sooner or 

 later die 2 . Leaves developed from the commencement in darkness remain 

 living for a longer period, but in this case also the weak power of carbon 

 dioxide assimilation, which a well nourished etiolated leaf may acquire, is 

 ultimately lost, as is also the power of turning green when exposed to light 

 (Ewart, 1. c., p. 554). The leaves of many plants are, however, comparatively 

 resistant and may develop to a marked extent in darkness as well as in an 

 atmosphere free from carbon dioxide, proving that the growth of the leaf 

 is not necessarily dependent upon a supply of auto-assimilatory products". 



1 Ewart, Journ. Linn. Soc., Vol. xxxi, 1896, pp. 430-38. 



* Vochting, Bot. Zeitung, 1891, p. 113. See also Ewart, Journ. Linn. Soc., Vol. XXXI, 

 P- 567 5 Jost, Pringsh. Jahrb., 1897, Bd. XXVII, p. 403. 



3 S. H. Vines, Arb. d. Bot. Inst. in YViirzburg. 1878, B.I. n. p. 114; Sachs. I.e., 1884, 



