824 FORMATION OF THE SECRETIONS OF PLANTS. 
2. Secondary products of metastasis, some of which, as sweet 
secretions, &c., are necessary for the perpetuation of the species, 
by attracting some insects, guarding against the visits of 
others which would be injurious, and so furthering fertilisation ; 
while some—as ethereal oils, resins, colouring matters, and many 
acids and alkaloids—appear to be of no further use to the plant. 
The important influences which these changes in the leaves 
have in promoting the purity of the atmosphere we breathe 
(page 802), the healthiness or otherwise of a particular country 
(page 798), and the fertility or barrenness of a soil (page 798), 
have likewise been already noticed. We have also seen that, in 
order that these changes may be properly performed, the leaves 
must be freely exposed to light; and from this dependence of 
assimilation on light, it follows, as we have noticed (page 804) 
that when the secretions of particular plants, which are other- 
wise agreeable, are injurious, or of unpleasant flavour, they can, 
by growing them in darkness or in diminished light, be made fit 
for the table, as is the case with Celery, Sea Kale, Lettuce, 
Endive, and others. For the same reason the plants of warm 
and tropical regions, where the light is much more intense than 
it is in this country or in other cold and temperate regions, are 
commonly remarkable for the powerful nature of their secretions, 
as is well illustrated by the stronger odours of their flowers, and 
the richer flavours of their fruits. (See also Electricity of 
Plants, page 858.) 
Again, as the formation of secretions depends upon the in- 
tensity of light, it frequently happens that when a plant of a 
warm or tropical region which naturally produces a secretion 
which may be of great value as a medicinal agent, or useful in 
the arts, is transported to this or any other climate in which the 
intensity of the light is much less than it is in its native country, 
the secretion is not formed at all, or in diminished quantity. 
Even if such plants be placed in our hot-houses, where they 
may he submitted to the same degree of heat which they obtain 
naturally in their native countries, their secretions are either not 
formed at all, or in diminished amount, because light is the 
main agent concerned in their formation, and we cannot increase 
the intensity of light as we can that of heat, by artificial means. 
Another cause which commonly interferes with the formation of 
the secretions of plants of warmer regions when grown in our 
hot-houses is the want of a proper and incessant supply of fresh 
air to facilitate transpiration, &c. 
The above facts are of great interest, as they have an impor- 
tant bearing upon the growth of plants and fruits for the table, 
as well as in a medicinal and economic point of view. At pre- 
sent, however, much remains to be discovered, before we can be 
said to have anything like a satisfactory explanation of the 
causes which influence the formation of the secretions of 
plants ; for it is found that the same species of plants when grown 
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