120 TRANSPORT OF WATER AND SOIL SOLUTES 



the thick and thin places in the walls the thick places to keep 

 the tubes from collapsing and the thin places to allow the inflow 

 and outflow of water and solutes. 



2. Study cross and longitudinal radial and tangential sec- 

 tions of pine wood, double stained and permanently mounted. 

 Study the cross section first and draw to scale a small portion 

 at the juncture of the early growth of one year with the late 

 growth of the preceding year where a medullary ray cuts through. 

 Look sharply for any means of interchange of materials between 

 early growth, late growth, and medullary rays. If the drawing 

 is not made as exactly as possible, important details will be left 

 out. 



Now study the longitudinal radial section at the juncture of 

 two rings of growth where a medullary ray runs across. Make 

 a careful drawing to the same scale as that of the previous draw- 

 ing. After studying the cross section the appearance of the 

 radial, section is surprising. What we now want to do is to 

 discover in one section every thing we have found in the other. 

 To do this it is necessary to recognize in the radial section both 

 the tangential and radial walls of the tracheids. In the cross 

 section these are easily recognized, since the radial walls run 

 parallel with the medullary rays and the tangential walls are per- 

 pendicular to the rays. Notice that in the radial section the 

 tangential walls are seen on edge and the radial walls are lying 

 flat. Every thing seen in the radial section can be found in 

 the cross section and vice versa but from different points of 

 view and having a different appearance, and the same thing 

 is true of the tangential section. Discover whatever means 

 of communication there is between the tracheids laterally and 

 longitudinally and between the ray cells and the tracheids and 

 between the two rings of growth. Are the medullary ray cells 

 all alike? 



3. Study the longitudinal tangential section and discover 

 from this point of view all details that have been found in the 

 other two. Look for provision for the flow of materials in 

 all directions. Draw to scale a small portion where a medul- 



