INFLUENCE OF LIGHT ON PLANTS. 63 



there is a free exposure to the atmosphere, through which carbonic acid 

 is diffused ; and hence, as fast as it is removed by decomposition, it is 

 restored by absorption. 



82. Here then are the conditions and materials ; what i# the result ? 

 As a consequence of the conjoint action of light and of a vegetable cell- 

 germ, with a moderate degree of heat, upon carbonic acid and water, 

 we find a vegetable structure produced, whose fabric chiefly consists of 

 carbon, united with the elements of water. Whether this union is really 

 as simple and direct as is implied by this expression, or whether the 

 same proportions of ogygen, hydrogen, and carbon are united in a dif- 

 ferent form, is not a matter of consequence to the present inquiry ; the 

 general fact being, that by the decomposition of the carbonic acid, oxy- 

 gen is set free, and carbon is made to unite with the elements of water ; 

 so as to form an organic compound, which is appropriated by the Vege- 

 table organism as the material for its growth. How far Light is also 

 concerned in the production of the proteine-compounds which are gene- 

 rated by Plants, not merely for the use of Animals, but also as part 

 of the material of their own growth, has not been yet ascertained ; but 

 it is probable that these are not the less dependent upon its agency for 

 their formation, since they are formed under the same circumstances 

 with the preceding. 



83. The process whose conditions we have thus examined, is carried 

 on in the individual cells, that compose the highest and most complex 

 Plants, precisely as in those which constitute the entire organisms of the 

 lowest. Thus if a few garden-seeds of any kind be sown in a flower-pot, 

 and be caused to germinate in a dark room, it will soon be perceived 

 that although they can grow for a time without the influence of light, that 

 time is limited ; the weight of their solid contents diminishes, although 

 their bulk may increase by the absorption of water ; their young leaves, 

 if any should be put forth, are of a yellow or gray-white colour, and 

 they soon fade away and die. But if these plants are brought out suffi- 

 ciently soon into the bright sunlight, they speedily begin to turn green, 

 they unfold their leaves, and evolve their different parts in a natural 

 way ; and the proportion of their solid contents goes on increasing from 

 day to day. If the fabric be then subjected to chemical analysis, it is 

 found to contain oxygen, hydrogen, carbon, and nitrogen; united in 

 various proportions, so as to form compounds that differ in the various 

 species, though some, such as gum, starch, cellulose, and albuminous 

 matter, are the same in all. If the plants be made to grow in closed 

 glass vessels, under such circumstances that an examination can be 

 accurately made as to the changes they are impressing on the atmo- 

 sphere, it is discovered that they are constantly decomposing its carbonic 

 acid, appropriating its carbon, and setting free its oxygen, so long 

 as they are exposed to the influence of sunshine or bright daylight. 

 They also appropriate a part of the minute quantity of ammonia which 

 is diffused through the atmosphere ; extracting its nitrogen to employ 

 it in the production of their azotized compounds. It is capable of being 

 demonstrated by experiment, that these changes are confined to the green 

 surfaces of plants, and therefore to the leaves or leaf-like organs, to the 

 young shoots, and to the stems of herbaceous plants, or of those in which 



