CIECTTLATION Ot SAP. 89 



change their functions and propel the fluids in other 

 directions through the cells instead of lengthwise with the 

 grain of the wood, as may be readily proved by removing 

 alternate sections of wood from the stem of a tree. Prof. 

 Lindley, in referring to the functions of the alburnum 

 and liber in trees, says: " The two have equally important 

 ofiBces to perform; the alburnum giving strength and 

 solidity to the stem and conveying sap upwards ; the 

 liber not only conveying sap downward, but covering over 

 the alburnum, protecting it from the air and enabling it 

 to form without interruption. It is therefore indispen- 

 sable to the healthy condition of plants that neither the 

 alburnum nor liber should be injured." 



The inner layers, or heart-wood, of trees are dead, and 

 they may be removed entirely without serious injury to 

 the living parts, as often occurs, and as seen in hollow 

 trees, which sometimes live for centuries in this condi- 

 tion, new layers of alburnum being annually added to 

 the outside. It is now quite generally conceded that the 

 annual increase in the diameter of exogenous stems is due 

 to the multiplication of the cells of the cambium layer, 

 and the material from which they are formed — or at least 

 the greater part of it — descends in the bark ; but there 

 have been, and still are, vegetable physiologists who deny 

 the existence of any distinct downward flow of organiz- 

 able matter through the Hber. Dr. J. M. Schleiden, in 

 his " Principles of Botany," emphatically denies any such 

 movement in plants, and says : " As water is continu- 

 ally exhaled by plants in proportion to the dryness, mo- 

 tion and warmth of the air, so the sap becomes concen- 

 trated, and thus interrupts the endosmatic process toward 

 the other cells ; this action is continued naturally down- 

 ward toward the roots, by which new watery and unas- 

 similated fluids are absorbed. If this stream of crude 

 sap is artificially interrupted in its course from below 

 upward, the sap in the upper part becomes more concen- 



