Three - year- old linden 



Cork layer — 

 Phloem ducta- 



^ Cambium 



Xylem 



ducts 



-Wood 



fibers 



Bast fibers 



/ 



Epidermis 



-Pith- 



■^ 



Pith ray- 



Left, © General Biological Supply House, Inc. 



STRUCTURE OF A DICOT STEM 



Growth in the cambium layer produces new woody tissue on the inside and new 

 bark tissue, or cork, on the outside of this layer. During the spring, when growth is 

 rapid, large xylem tubes are formed. Later, growth slows down, and a definite ring 

 of denser tissue is formed. The number of annual rings in the woody part of the 

 stem tells us the age of a tree. Food travels down the stem from the leaves through 

 the phloem tubes; water and dissolved mineral salts travel up from the roots through 

 the xylem tubes. Rays of pith cells connect the cambium with the xylem tubes 



as due to osmosis, and transpiration were sufficient to explain the rise of sap 

 (see illustration, p. 148), 



The minute diameters of the xylem vessels probably also play a part in 

 connection with osmosis and transpiration. No vessels reach the whole 

 length of a plant, so that the "capillary" attraction can raise water but a short 

 distance in each cell. Other experiments have shown that water is ''pulled" 

 through the xylem tubes as it evaporates from the cells of the leaves. This 

 is explained by the fact that particles of water cohere, or cling together, when 

 confined in the narrow tubes. The network of water-threads in the plant 

 can carry a considerable amount of strain, equal to a pull to the top of the 

 tallest trees. 



Fluids in plants not only rise, but, as we have seen, move also from the 

 leaves toward the roots. We can show that this part of the circulation is 

 by way of the phloem vessels. If the bark is removed from a tree so as to 

 leave a complete ring or "girdle" unprotected, the tree can continue to live 

 for the rest of the season. This shows that the water continues to rise from 



147 



