THE FUNCTIONS OF THE WOOD 207 



of maltose in the sap appears to indicate that this sugar, 

 arising from the action of diastase on starch, is itself 

 further hydrolyzed to glucose to a great extent. This is 

 to be regarded as evidence of the presence of maltase in 

 the cells. 



The whole subject is one of great interest, and further 

 and more accurate analyses of the sugars are necessary 

 before any degree of certainty can be reached. 



Before closing it may be profitable to consider the dis- 

 tribution of the elements of the wood with regard to its 

 bearing on the co-ordinated functions of the living cells 

 and of the tracheae. 



THE LIVING AND NON-LIVING ELEMENTS OF THE WOOD. 



As is well known, all woody stems are traversed by 

 medullary rays, which place the wood and the bast in close 

 communication. In conifers the wood consists of spindle- 

 shaped tracheids with bordered pits, and with (e.g., Taxus) 

 or without (e.g., Pinus) internal spiral thickenings. In 

 other types of wood there are, in addition, wood parenchyma 

 cells, and vessels formed by the union of elongated tubular 

 cells, the end walls of which are absorbed in many places. 

 In such stems a great part of the transpiration stream 

 appears to travel in the vessels, though a portion may 

 traverse the wood fibres, which are thick-walled tracheids. 

 Thickenings, spiral, reticular, scalariform, or annular, are 

 to be found in the conducting elements, and pits are 

 situated on the thinner portions of the cell walls. 



The medullary rays consist mainly of living cells, though 

 in some, especially in the larger types of ray, there are 

 tracheids. Many of the cells are seen to be loaded with 

 starch grains and other reserve materials. These may fre- 

 quently be observed in the wood parenchyma cells also. 



The immediate source of supply for the medullary ray 



