THE STEM 



115 



T27. 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, y^ |o / 

 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 

 movement of the different substances. 



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 



Fig. 131. — Diagram show- 

 ing general movement of sap. 



