THE STRUCTURE AND GROWTH OF STEMS 175 



they are by live tissue and by bark outside of that, the 

 wood cells may be preserved indefinitely. Their death 

 means only that their protoplasts cease to exist. 



The reason why dead tracheary vessels are of more 

 service to the plant than hve ones is not difficult to under- 

 stand. The principal use of these cells to the plant is 

 that they furnish the path for the ascending water, the 

 transpiration stream. This water, and the solutes which 

 are in it, can move much more rapidly through dead cells 

 than it can through living ones. You recall that when 

 water and solutes pass through living cells, they do it by 

 osmosis, being independent of one another in rate and direc- 

 tion of movement. You remember that the osmotic move- 

 ment of any solute depends upon whether the cell walls 

 and the protoplasmic membranes are permeable for that 

 particular solute, and upon differences in its abundance ; 

 that is, upon differences in osmotic pressure. Movement 

 of this character, taking place through living cells, is slow. 

 But movement through dead cells may be relatively fast. 

 Through dead tracheary vessels, water and solutes may 

 move together for considerable distances without having 

 to pass the barrier of a single protoplasmic membrane, and 

 they move much more rapidly than when moving by 

 osmotic 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." (Cowles's Ecology.) 



The most important conducting cells of the phloem or 

 bast are the sieve vessels. (See Figure 61.) It is, through 

 these sieve vessels that food principally moves. They are 

 called sieve vessels or sieve tubes because their end walls 

 are full of holes like a sieve. Side by side with the sieve 



