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 pui'poses." 



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- 



