so 



PHYSIOLOGY. 



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 jjath of tlie 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 fibro-vascular bundles. — We should now make quite 

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

 examination, (jr tliey ma\' be made \\\\\\ a microtome and mounted in Canada 

 balsam, and in this condition will answer for future stud}'. To illustrate the 

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

 bean. On examining these cross sectii^ns \\'e 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 



tig. 58. 



Xylem portiun uf bundle. Cambium portion of bundle. Basl portion of bundlu 



Section of vascular bundle of sunflower stem. 



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

 end directed toward tlic center of the stem. If we look at the auction 

 as a whole we see that there is a narrow continuous ring ■^" of small cells 



* This ring and the bundles separate the stem into two rei;ions. an outer 

 one composeil of large cells with thin walls, known iis the cortical cells, or 

 collectively the ccr/tu-. The inner portion, corrrspDinliug tcwhat is called 

 the pith, is made up of the sanie kiu<] of cells and is eidled the i)u\h(lla. or 

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

 the tissue is c;dled piirencln ma. Parenchyma belongs to the group ul 

 tissues called fuudainnitah 



