THE STEM 115 
127. Ringing fruit trees. — The course of the sap explains 
why farmers sometimes hasten the ripening of fruit by the 
practice of ringing. As the food material cannot pass below 
the denuded ring, the parts above become gorged, and a pro- 
cess of forcing takes place. The practice, however, is not to 
be commended, except in rare cases, as it generally leads to 
the death of the ringed stem. The portion below the ring 
can receive no nourishment from above, and will gradually 
be so starved that it cannot even act as a carrier of crude 
sap to the leaves, and so the whole bough will perish. 
128. Sap movement not circulation.— It must not be 
supposed that this flow of sap in plants is analogous to the 
circulation of the blood in animals, 
though frequently spoken of in pop- 
ular language as the “ circulation of 
the sap.” There is no central organ 
like the heart to regulate its flow, and 
the water taken up by the roots does 
not make a continual circuit of the 
plant body as the blood does of ours, 
but is dispersed by a process of general 
diffusion, partly into the air through 
the leaves and partly through the plant 
body as food, wherever it is needed. 
Figure 131 gives a good general idea 
of the movement of sap in trees, the $ 
arrows indicating the direction of the — Fic. 131.— Diagram show- 
movement of the different substances. 2 #°net=! movement of sap. 
129. Unexplained phenomena.— Though the forces 
named above undoubtedly exert a powerful influence over 
sap movement, their combined action has not been proved 
capable of lifting the current to a height of more than 200 
feet, while in the giant redwoods of California and the tower- 
ing blue gums of Australia, it is known to reach a height of 
more than 400 feet. The active force exerted by the cell 
protoplasm has been suggested as an efficient cause, but as 
