STEMS 695 



conduction, but it is much more probable that it is a layer in which 

 surplus starch accumulates. Much of the carbohydrate that descends 

 in the cortex and leptome during the summer and autumn may ascend 

 in the wood with water and soil salts the following spring, as in the sugar 

 maple, whose sap may contain as much as three per cent of sugar. 



Advantages associated with various bundle arrangements. — The 

 grouping of conductive elements into strands is advantageous, partly 

 because of the consequent economy in space and structural material in 

 the protective stereome as well as in the hadrome and leptome, but more 

 particularly because continuity thus is made possible. In the bundle the 

 hadrome seems to have the place of advantage. In the stem it is inside 

 the leptome, where there is greater freedom from mechanical strains, 

 and where dangers incident to transpiration are more remote. In the 

 leaf (even in ferns, where the stem bundles are concentric) the hadrome 

 is uppermost, and therefore closest to the most active food-making re- 

 gion. Even in roots the hadrome occupies equally with the leptome 

 the place of advantage, that is, nearest the cortex, from which food 

 materials come and to which organized foods go. 



There has been advanced the very dubious hypothesis that a " struggle for ex- 

 istence " has taken place between the tissues, and that the stronger element, the 

 hadrome, has "won" the place of advantage. In secondary tissues it is highly 

 advantageous for the xylem, which gives rise to the permanent tissue, to be inner- 

 most, since the less permanent tissue may then exfoliate as the other develops. 

 Where other conditions obtain, as in some lianas (e.g. Mucuna) and in monocotyls (as 

 Cocos), temporary and permanent tissues are intercalated in such a way as to make 

 impossible the development of a solid cylinder of permanent tissue; in Cocos the 

 decay of the temporary elements leaves the wood in disconnected strands, whence 

 the name, porcupine wood. 



The slight reduction of leptome in hydrophytes is advantageous, for while sub- 

 mergence results in reduced water conduction, protein conduction is as prominent as 

 ever. The large development of leptome in lianas having bicoUateral bundles may 

 facilitate protein conduction in the long slender stems just as the elongated and 

 enlarged hadrome elements facilitate water conduction. The great development 

 of hadrome in xerophytes appears disadvantageous rather tban advantageous. 



In secondary wood, conduction is mainly in the alburnum, where living paren- 

 chymatous cells are intercalated among the dead vessels, the medullary rays fur- 

 nishing lateral connections between the alburnum, cambium, and phloem. The 

 dead duramen often is a region of accumulation of waste products (p. 725), the 

 vessels being filled with tannins, gums, and other excreta. Sometimes vessels are 

 closed by tyloses which arise through the intrusive growth of neighboring paren- 

 chyma cells. Tylosis formation 'characterizes especially the transformation of 

 alburnum to duramen, representing the final phase of activity in the wood paren- 



