300 ASSIMILATION. 



is shown in the following table (taken from the comprehensive 

 S3*nopsis in Watts's dictionary) : 



Cubic centimeters 

 in each liter of water. 

 Loch Katrine (Scotland) .3 



Bala Lake (Wales) 1.1 



Khine at Strasburg 7.6 



Ehone at Geneva 8.4 



Thames at Kew 50.3 



All the free carbonic acid dissolved in water can be expelled 

 by boiling. 1 



801. Absorption of carbonic acid by laud plants. These, with 

 their foliage exposed to the air, obtain from that source all their 

 supply of carbonic acid. No carbonic acid is taken up by their 

 roots : 2 the supply enters the plant through the younger epider- 

 mal tissues, chiefly, of course, that of the leaves. By the process 

 of respiration within the plant (see Chapter XI.) a small but ap- 

 preciable amount of carbonic acid is produced, and a part of this 

 is doubtless appropriated directly by the plant for the process 

 of assimilation. 



802. Carbonic acid and other gases found in the atmosphere 

 sustain to vegetable membranes certain relations which must 



1 According to Bunsen (Jahresb. der Chemie, 1853, p. 317), one volume of 

 water absorbs at 760 mm. barometric pressure, and at the temperatures noted, 

 the following amounts of various gases : - 



32C. 19.6C. 



Nitrogen 02189 vol 01515 vol. 



Oxygen 04553 " 03253 " 



Carbonic acid . . 1.5184 " 8545 " 



According to the same authority, these gases occur in rain-water in the fol- 

 lowing relative proportions : - 



C. 10 C. 20 C. 



Nitrogen ... 63.20 ... 63.49 ... 63.69 

 Oxygen .... 33.88 . . . 34.05 . . . 34.17 

 Carbonic acid . . 2.92 ... 2.46 ... 2.14 



2 This appears to be settled by the results of experiments made by Moll: 



( 1 ) when carbonic acid is afforded, even in excess, to shoots, whose leaves are kept 

 in an atmosphere free from carbonic acid, no formation of starch takes place ; 



(2) if such leaves are in the open air, the formation of starch is not increased 

 above its normal rate; (3) when carbonic acid is supplied to roots of plants 

 whose leaves and shoots are kept in an atmosphere free from carbonic acid, no 

 formation of starch takes place. If the leaves and shoots of such plants 

 are in the open air, there is no increase of starch above the normal amount 

 (Arbeiten des bot. Inst, in Wiirzburg, 1878, p. 113). 



