804 FORMATION OF ORGANIC COMPOUNDS BY LEAVES, 
few plants rendering the air of close rooms unwholesome by 
their action, is altogether erroneous. It is certain, however, 
that, under such circumstances, the odours of plants may affect 
injuriously, to some extent at least, individuals of ‘delicate 
organisation or peculiar idiosyncrasies. 
(4) Formation of Organic Compounds by Leaves.—By the 
alterations produced in the watery contents of the green leaves, 
&c., by exposure to light, heat, and air, the matters which they 
contain are left in a very active chemical condition or in a state 
prone to change, and therefore freely combine together. ‘By 
this means the different organic compounds are produced which 
are concerned in the development of new tissues ; and in the 
formation of others, such as resinous matters, various acids, 
numerous alkaloids, colouring matters, &c., which, so 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 se- 
cretions, or removed altogether from the plant as excretions. 
(See page 792.) The production of these organic substances 
takes place by assimilation, and metastasis. (See page 823.) 
We see, therefore, that without leaves cr 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, moreover, no woody matter is then formed 
(page 794), and but few of its peculiar secretions. The 
experiments of Pringsheim tend to show that the earliest 
nutritive product produced by the influence of light, heat, and 
air is formed in the interior of the chlorophyll grains. This 
principle he has termed hypochlorin, and by its oxidation he 
believes that all nutritive bodies, such as starch, glucose, and 
oil, are formed. It is also supposed by Pringsheim that the 
function of the green colouring matter is to act as a screen, and 
to reject the rays of the spectrum favouring oxidation, and to 
allow those only to pass which aid nutrition. It is possible, 
however, that the hypochlorin is simply the result of the action 
of the hydrochloric acid used in the experiment upon the 
chlorophyll. 
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 the tissues formed are seen to become etiolated, and ultimately 
to die ; or when potatoes are reared with a diminished supply of 
light, as in an orchard, or under trees, when the tubers are 
found to be watery in consequence of the small quantity of starch 
then produced. Another illustration of the effect produced by 
the absence of light is affordedin growing certain vegetables for 
Ss. 
