CHAP. XII. 
DIGESTION. 
383 
quantity of carbon fixed in the system during its growth. 
Thus we have every reason to believe that the 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 w'hich 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 intercepted 
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 
horticultural 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 AthencBum 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 
