NUTRITION. 51 



but the question of the channels through which 

 it is propelled is by no means one to which an 

 indisputable answer can be given. " The great 

 difficulty," says Professor Henslow, " in deter- 

 mining the precise channel through which the 

 progression of the sap takes place, must be as- 

 cribed to the perfect transparency of the vege- 

 table membrane, and the extreme minuteness of 

 these organs themselves. By placing a branch 

 in coloured fluids, such as a decoction of Brazil 

 wood or cochineal, they are absorbed and the 

 course of the sap through its whole passage into 

 the leaf may be regularly traced ; but on exam- 

 ining microscopically the stains which have been 

 left, it is scarcely possible to feel satisfied whether 

 they are on the outer or inner surface of the 

 vessels and cells which they have discoloured. 



Since there are many plants which 



possess no vascular structure, in them at least 

 we must allow the cellular tissue to be the true 

 channel through which the sap is conveyed . . . 

 .... The probability seems to be, that the 

 crude sap rises, at least in woody stems, through 

 the intercellular passages, where it bathes the 

 surface of the cells and vessels, all of which are 

 so many distinct organs destined to act upon 

 it." (Henslow's Principles of Botany, p. 179.) 



