330 THE FOOD OF PLANTS 



source of carbonic acid gas is available. Indeed the amount derived from 

 the roots may, when transpiration is active, be sufficient to prevent the 

 chloroplastids in the more deeply situated tissues at the base of a green 

 stem from losing the power of assimilating carbon dioxide when exposed 

 to light for prolonged periods in an atmosphere free from this gas l . 



Even a feebly developed cuticle is usually so impermeable that the 

 leaves can absorb practically no carbonic acid gas from the surrounding air 

 when the stomata are closed, so that any closure of the latter induced 

 by excessive transpiration causes assimilation almost entirely to cease. 

 Hence the formation of starch is in most cases entirely suppressed under 

 such circumstances, as it is also when the leaf-surface bearing stomata is 

 smeared with fat, whereas if only a portion of the leaf is so smeared, 

 starch appears for the most part or entirely in the unsmeared regions -. 

 Blackman has shown that hardly any carbonic acid gas penetrates through 

 the non-stomatic surface of a leaf, whereas when assimilation is active 

 the stomatic surface robs the air drawn slowly over it of almost all its 

 carbon dioxide 3 . 



Air currents continually bring fresh supplies to the leaves, and hence 

 large quantities can be assimilated, although but 0-03 to 0-04 per cent, of 

 this gas is present in the atmosphere 4 , so that 4,000 litres of air contain 

 the 1,250 cubic centimetres (about 2-5 grammes) of carbon dioxide that 

 a surface of one square metre of actively assimilating leaves may decom- 

 pose per hour (Sect. 55). A comparative research showed that a similar 

 area of a 7-5 per cent, solution of soda absorbed carbon dioxide with 

 five to six times greater avidity when air was drawn slowly over it. 



Currents of water are of the utmost importance in providing aquatic 

 plants with continual supplies of dissolved carbonic acid. Owing to 

 the marked solubility of the latter a given volume of water can hold 

 a higher percentage than is present in the supernatant air, while still 

 more may be held in the form of dissolved carbonates and bicarbonates 5 . 

 Indeed, according to Hassak, a green plant is not only able to obtain carbon 



1 Ewart, Journ. Linn. Soc. Bot., 1896, xxxi, p. 569. 



- Stahl, Bot. Zeitung, 1894, p. 129. Moll (Landw. Jahrb., 1877. Bd. vi, p. 343; obtained 

 a similar result by exposing a portion of a leaf to light in a CO 2 -free atmosphere, the other portion 

 being in an atmosphere containing this gas. 



3 Blackmann, Phil. Trans., 1895, Vol. cxxvi, p. 556 ; Annals of Botany, 1895, Vol. ix, p. 164. 

 When the percentage of CO 2 is high, more passes through the cuticle, and to this fact the remarkable 

 exceptions noticed by Boussingault are due (Agron., &c., 1868, T. iv, p. 359). Cf. also Stahl, 1. c., 

 p. 132. Researches on the removal of carbon dioxide from air drawn slowly over assimilating leaves 

 were carried out by Boussingault, Die Landw. , 1851, Bd. I, p. 40; Vogel und Witwer, Abhand- 

 lungen d. Munch. Akad., 1852, Bd. vi, p. 267; Corenwinder, Ann. d. chim. et d. phys., 1858, 

 iii. ser., T. LIV, p. 321. 



4 Cf. Sachsse, Agriculturchemie, 1888, p. 10. 



' See Sachsse, Agricullurchcmie, 1888. p. 9. Cf. also Fr. Darwin and Pcrtz, Proc. Camb. Phil. 

 Sue., 1896, Vol. ix, p. 76. 



