STEMS 693 



gation of the hadrome elements probably facilitates conduction, move- 

 ment through elongated lumina being supposed to be more rapid than 

 through a series of short lumina separated by walls. Tracheae thus 

 may be better conductive elements than tracheids, though the latter 

 certainly are efficient, since they alone are present in the conifers, which 

 include the tallest known trees. In some conifers, as the larch, con- 

 duction has been shown to be essentially as rapid as in dicotyls. Per- 

 haps the conductive efficiency of conifer wood is due in part to its 

 relatively long tracheids. Doubtless the large caliber of the hadrome 

 elements also facilitates conduction. The large and long hadrome ele- 

 ments of lianas may be regarded as very advantageous, since the length 

 of the stems is so great in proportion to their diameter. Oblique end 

 walls have been thought to be more advantageous than horizontal end 

 walls because they present a greater diffusion surface. 



The advantages of differential lignification. — Ligni6cation is highly beneficial, 

 since lignin, which is the chief factor in giving rigidity and strength to woody tissues, 

 is at the same time permeable to water and solutes. The slightly lignified spiral 

 and annular vessels are not so strong and rigid as are the larger and more lignified 

 . pitted, reticulated, and scalariform vessels, which may be regarded as the main 

 conductive elements. Though the entire wall is permeable, the thin spots or pits 

 probably represent more permeable regions which facilitate rapid lateral transfer, 

 without interfering with the mechanical efficiency of the wall. Bordered pits are 

 supposed to act somewhat in the manner of a valve, opening when the pressure on 

 the two sides is equal and closing when it is unequal, a pressure difference of one 

 fifteenth of an atmosphere being sufficient to cause closure; thus bordered pits may 

 either facilitate or impede lateral transfer. 



The advantages of dead tissues in conduction. — One of the chief 

 advantages of tracheids and tracheae is associated with their early 

 death. Water and solutes pass through living cells by osmosis, their 

 rate and direction of movement being independent of one another; the 

 movement of any given solute is conditioned by the permeability of the 

 cell walls and the protoplasmic membranes for that particular solute, 

 and by differences in its pressure in adjoining cells. In the dead had- 

 rome, however, water and solutes may move together for great distances 

 without passing protoplasmic membranes and only occasionally trav- 

 ersing cell walls, movement being much more rapid than by diffusion 

 through living cells. Except for a few cells in the epidermis and cortex 

 of the root and in the mesophyll of the leaf, the entire course of water 

 through a plant is in dead tissues. The streaming of protoplasm, a 



