CHAP. X. 
DIGESTION. 
319 
brittle Wainscot Oak of the Black Forest is produced by the 
very same species as produces the tough and solid naval timber 
of Great Britain. Starch, again, in which carbon forms so large 
a proportion, and which, in the Potato, the Cassava, Corn, 
and other plants, ministers so largely to the nutriment of man, 
depends for its abundance essentially upon the presence of 
light. For this reason. Potatoes grown in darkness are, as 
we say, watery, in consequence of no starch being developed in 
them; and the quantity of nutritious, or amylaceous matter 
they contain is in direct proportion to the quantity of light to 
which they are exposed. For this reason, when orchard- 
ground is under-cropped with Potatoes, the quality of their 
tubers is never good ; because the quantity of light intercept- 
ed by the leaves and branches of the orchard-trees prevents the 
formation of carbon by the action of the sun's rays upon the 
carbonic acid of the Potato plant. Mr. Knight has turned 
his knowledge of this unquestionable fact to great account in 
his application of the principles of vegetable physiology to hor- 
ticultural purposes." 
That the intensity of light does in fact vary most mate- 
rially in different climates, is a matter of inference from the 
difference of temperature. But it never has been actually 
measured, to my knowledge, by any one except Herschel, 
who, in a communication made to the " Athenaeum" news- 
paper of April 25. 1835, speaks of an instrument called an 
actinometer, which he finds extremely sure and uniform in 
its indications. This instrument gives the force of sunshine 
at the Cape of Good Hope as 48°75, while ordinary good 
sunshine in England is only from 25° to 30°. 
The principal part of the secretions of plants is deposited in 
some permanent station in their system; as in the roots of 
perennials, and the bark and heartwood of trees and shrubs. 
It appears, however, that they have, besides this, the power of 
getting rid of superfluous or deleterious matter in a material 
form. In the Limnocharis Plumieri there is a large pore ter- 
minating the veins of the apex of the leaf, from which water is 
constantly distilled. The pitchers of Nepenthes, which are 
only a particular kind of leaves, secrete water enough to fill 
half their cavity. But, besides this more subtle fluid, secre- 
