44 



PHYSIO LOG Y. 



97. We thus see that instead of the liquids passing through 

 the entire stem they are confined to definite courses. Now that 

 we have discovered the path of the upward movement of water 

 in the stem, we are curious to see what the structure of these 

 definite portions of the stem is. 



98. Structure of the flbro-vasoular bundles. — We should now make quite 

 thin cross sections, either free hand and mount in water for microscopic 

 examination, or they may be made with a microtome and mounted in Canada 

 balsam, and in this condition will answer for future study. To illustrate the 

 structure of the bundle in one type we may take the stem of the castor-oil 

 bean. On examining these cross sections we see that there are groups of 

 cells which are denser than the ground tissue. These groups correspond to 

 the colored areas in the former experiments, and" are the vascular bundles 



Fig. 4 i. 



Xylem portion of bundle. Cambium portion of bundle. Bast portion of bundle. 



Section of vascular bundle of sunflower stein. 



cut across. These groups are somewhat oval in outline, with the pointed 

 end directed toward the center of the stem. If we look at the section as a 

 whole we will see that there is a narrow continuous ring * of small cells 



* This ring and the bundles separate the stem into two regions, an outer 

 one composed of large cells with thin walls, known as the cortical cells, or 

 collectively the cortex. The inner portion, corresponding to what is called 

 the pith, is made up of the same kind of cells and is called the medulla, or 

 pith. When the cells of the cortex, as well as of the pith, remain thin walled 

 the tissue is called parenchyma. Parenchyma belongs to the group of 

 tissues called fundamental. 



