OF THE LEAVES. 741 



effect upon the atmosphere in which they are placed. It might 

 be readily shown that it would require some thousands of plants, 

 in this way, to vitiate 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 this action, is altogether erroneous. It is certain, however, 

 that the odours of plants may affect injuriously, to some extent 

 at least, certain individuals of delicate organization or peculiar 

 idiosyncrasies. 



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 the Products and Secretions hy Leaves. — 

 By the changes produced in the watery contents of the green 

 leaves by exposure to air and light, as noticed in the three 

 preceding sections, the materials which they then contain 

 are in a very active chemical condition, and freely combine 

 together, by which various substances are formed, such as 

 starch, sugar, gum, proteine matters, &c., which are directly 

 concerned in the growth and nutrition of the plants in which 

 they are found; 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 acccordingly removed from the young and 

 vitally active parts, and either stored up in the older tissues of 

 the plants as secretions, or removed altogether from them as 

 excretions. The production of these substances 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; and that with- 

 out the exposure of even the leaves to light, no proper assimila- 

 tion of the various matters taken up by the plant could take place; 

 and hence, if a plant be put into the dark, it becomes blanched 

 {etiolated), in consequence of the non-development of chloro- 

 phyll, and that, moreover, 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 under a diminished supply of light, as when they are 

 grown in an orchard, or under trees, under which circumstances, 

 the tubers are what is called watery, in consequence of the small 

 quantity of starch then produced. 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 

 3b4 



