no 



TRANSPORT OF WATER AND SOIL SOLUTES 



Tangential Wall 



Fig. 55. — Diagram to show 

 the radial flow of water in pine 

 wood, from the tracheids of 

 the late growth of one year 

 into those of the early growth 

 of the succeeding year. 



is seen that the pits are plentiful in the slanting common wall 



between two vertically contiguous tracheids (Fig. 56). 



Relation of the Tracheal Tissues 

 to the Medullary Rays and Wood 

 Parenchyma. — One of the functions of 

 the medullary rays is to carry radially 

 into the bark water which they have 

 taken on from the tracheal tubes and 

 tracheids. Their other most important 

 function of transporting and storing 

 food will be spoken of in later chapters. 

 Wherever a medullary ray comes into 

 contact with a tracheal tube or tracheid 



there are pits or thin areas in the common wall separating them 



through which water and solutes can the more easily pass. 



And the same condition exists where the wood 



parenchyma cells come into contact with the 



tracheal tubes and tracheids. The medullary 



rays and wood parenchyma are therefore in 



position to take up and store water, and to 



assist in its radial and tangential distribution. 

 It has not been demonstrated, and it is 



indeed doubtful, that the rays and wood 



parenchyma as living tissues assist mate- 

 rially the dead tracheal tissues in the vertical 



transmission of water. The undoubted use 



of their intimate relationship, aside from the 



radial and tangential distribution of water, 



will be told in subsequent chapters. 

 The Ring of Annual Growth. — The 



physiological significance of the ring of 



growth has already been told in the chapter 



on secondary increase in thickness. Recapit- 



ulating in a sentence: The large tracheal 



Fig. 56. — Diagram 

 indicating by arrows 

 how the water in the 

 tracheids of pine passes 

 longitudinally from one 

 tracheid to another. 



tubes and tracheids of the early growth provide for the in- 

 creased demand for water, while the predominating wood fibers 



