1 64 



THE STEM PROPER 



308. — Diagram showing gen- 

 eral movement of sap. 



perish. Figure 308 will give a good general idea of the 

 movement of sap in trees, the 

 !» / arrows indicating the direction of 



the movement of the different 

 substances. 



233. Sap Movement not Circula- 

 tion. — 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 popular 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 pro- 

 cess of general diffusion, part into the air through tran- 

 spiration, and part through the plant body as food, 

 wherever it is needed. 



234. Unexplained Phenomena. — While root pressure will 

 account for the rise of sap to a certain extent, none of the 

 causes assigned by physiologists are sufficient to explain all 

 the phenomena. The highest force as yet proved to be 

 exerted by it is sufficient to balance a column of water 

 only ten to fifteen meters (thirty to fifty feet) high. The 

 power with which it acts seems to vary in different plants. 

 In the nettle it is capable of lifting the sap to a height of 

 about 4.5 meters (15 feet) and in the grapevine more than 

 II meters, or about 36.5 feet. It is claimed that in the 

 birch it exerts a lifting force nearly equal to the pressure 

 of a column of water eighty-five feet high, but even this is 

 quite inadequate to explain the rise of sap to the tops of 

 trees three hundred and four hundred feet high, Hke the 

 giant redwoods of California or the still taller blue gums of 

 AustraUa. Capillary attraction and the buoyant force of 



