ORGANS OP CIRCULATION. 99 



Preparation of the Pulp. 



As the blood in animals is propelled from the heart 

 to the farthest surface of the skin and of the lungs, so 

 is the sap of plants propelled from the spongelets of 

 the roots to the farthest surfaces of the leaves, where it 

 gives off a portion of its superfluous water, in the pro- 

 portion of two-thirds, as the animal blood does at the 

 surface of the skin and of the lungs, provided always 

 the pores of the leaves be open ; and this depends in a 

 great measure on the presence of sunlight. Conse- 

 quently, as exhalation cannot take place at night or in 

 cloudy or very cold weather, the sap must accumulate, 

 and its rise must cease, or be greatly diminished. 



The watery vapour thus given off may be obtained 

 by enclosing the branch of a vine or other plant in a 

 glass receiver: it is limpid and apparently pure, but 

 putrifies sooner than common water. 



The third of the sap, or the portion which remains 

 after the evaporation of the two-thirds, more or less, 

 as just explained, is supposed to undergo other changes 

 in the leaf, though these are not well understood, and 

 under the name of vegetable blood, proper juice, or 

 better, as I think, pulp J , returns into the body of the 

 plant 



Descent of the Pulp, 



Dr. Darwin immersed plants of spurge in red fluids, 

 which he saw rise through the leaf-stalk into the leaf, 



(1) In Latin, Cambium. 

 H2 



