OF THE LEATES. 755 



in this way, to Titiate the air of a room to anything like the 

 extent of that of a single animal, and that, therefore, the idea 

 of a few plants rendering the air of close rooms unwholesome 

 by their action is altogether erroneous. It is certain, however, 

 that the odoiu's of plants may affect injuriously, to some extent 

 at least, certain individuals of delicate organization or peculiar 

 idiosyncrasies. 



The decomposition of carbonic acid is probably effected by 

 the influence of chlorophyll: when leaves are not green, as is 

 the case in many parasitic plants and in those which are more 

 or less blanched, they, like the other parts of a plant in a similar 

 condition, exhale carbonic acid. 



4. Formation of Organic Products and Secretions hy Leahies. 

 — By the changes produced in the watery contents of the green 

 leaves by exposure to air and light, the materials which it con- 

 tains are in a xerj active chemical condition or in a state prone 

 to change, and therefore freely combine together. By this 

 means various substances are formed, such as starch, sugar, 

 gum, proteine matters, &c., which are directly concerned in 

 the growth and nutrition of the plant ; as well as others, such 

 as resinous matters, various acids, numerous alkaloids, colouring 

 matters, &c., which, as far as we know at present, perform no 

 further active part in the plant, and are accordingly removed 

 from the young and vitally active parts, and either stored 

 up in the older tissues as secretions, or removed altogether from 

 the plant as excretions. The production of these organic sub- 

 stances is commonly termed Assimilation. We see, therefore, 

 that without leaves or other analogous green organs no growth 

 to any extent could take place, or any peculiar secretions be 

 formed ; but it must be also recollected that without the exposure 

 of even the leaves to light, no proper assimilation of the various 

 matters taken up by the plant can be effected ; for instance, if a 

 plant be put into the dark, it becomes blanched {etiolated), in 

 consequence of the non-development of chlorophyll, and, more- 

 over, no woody matter is then formed, and but few of the peculiar 

 secretions. The effect of the absence of light upon plants is 

 well shown when a potato tuber sprouts in the dark, in which 

 case the whole of its tissues are seen to become etiolated, and 

 ultimately to die ; or when potatoes are reared with a diminished 

 supply of light, as when they are grown in an orchard, or under 

 trees, under which circumstances the ti\bers are fotmd to be 

 watery, in consequence of the small quantity of starch then pro- 

 duced. Another illustration of the effect produced by the 

 absence of light is afforded in growing certain vegetables for the 

 table, such as Sea-Kale, Celery, &c. In these latter instances, 

 when the plants are grown freely exposed to light, they form 

 abundance of woody matter, which renders them tough or stringy ; 

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