248 SUPPLY OF CARBON TO PLANTS. 



tact with plants, removing that which had given up its carbonic 

 acid, and bringing new supplies from the upper or lower air. It has 

 also been proved that plants grow faster in agitated than in still 

 air. (Boussingault, p. 42.) This effect of winds is general ope- 

 rating with nearly equal benefit on all neighbouring localities } and 

 this also of course is not within man's control, or even under his 

 partial direction. 



There is still another mean, by which possibly the desired end 

 may be attained. Though we cannot increase the supply of car- 

 bonic acid, or bring more of the actually existing supply in con- 

 tact with the leaves of plants, yet if we can stimulate the plants to 

 attract, seize upon, and rapidly absorb the contiguous carbonic 

 acid, instead of the much greater part passing by and escaping from 

 the otherwise feebler attracting powers of plants, then the same 

 object would be effected as if by actual increase of the supply of 

 carbonic acid. There is good reason to believe that such greater 

 stimulation of the appetite of plants and increased power of taking 

 up carbonic acid is to be conferred by the application of vatious 

 manures ; but more especially and in greater measure by the use 

 of calcareous manures ; as I shall endeavour to show. , 



Universal as is this function of growing plants of absorbing and 

 fixing the carbon of the atmosphere essential as it is to their 

 existence and largely as it is exercised to the extent of thereby 

 obtaining much the larger part of one-half the whole dry weights 

 of plants still this power is strictly limited by, or only exerted 

 under, certain known conditions. It is by their green matter only 

 that plants absorb carbonic acid, and that under the stimulating 

 influence of light. Through all the day, and by all their leaves 

 and other green parts, plants are absorbing carbonic acid from the 

 air, and assimilating and fixing its carbon, and evolving the oxygen 

 gas, the other constituent element of the carbonic acid. But this 

 operation always ceases with the withdrawal of light ; and even a 

 reverse operation, to smaller extent, proceeds during the night, 

 when the leaves actually evolve some of the larger quantity o car- 

 bonic acid which had been absorbed during the previous daylight. 

 It is well known that any plant, or single brand), of a plant, 

 secluded from light, does not acquire the usual green colour, but 

 remains white. In this state, the white leaves and stems exert 

 very little power, if any, in absorbing carbon. If the whole of 

 any plant is kept in the dark during its growing state, it must 

 soon die, for want of this essential source of sustenance. 



1. Calcareous earth causes plants to draw more carbon from the 

 atmosphere. 



The vigorous growth of plants, and the intensity or depth of 

 their green colour, always go together and in proportion to each 



