COMPARATIVE STUDY OF THE STEM 109 



while others have clevelu])e(l lo xarioiis tyj)es of tricliomes, includ- 

 ing glandular capitate hairs and multicellular unbranched hairs, 

 the latter similar to the hairs already studied on the geranium 

 leaf. (Refer to Exercise V.) 



(h) Immediately within the epidermis is found the cortex, a 

 region composed of twelve or more layers of rather typical "iso- 

 diametrie" parenchyma cells which are separated from one an- 

 otlier by prominent intercellular air spaces. A protoplast is 

 present in many of these cortical cells, the main functions of 

 which are photosynthesis and food storage, as is evidenced by the 

 frequency of starch grains in these cells ; druses of calcium oxylate 

 are found in many of the cortical cells, occupying nearly the 

 whole cavity of the cell. 



Depending largely on the distance from the shoot apex at 

 which the sections on your slide were taken, j^ou will find the 

 early or later stages in the development of cork or phellem. In 

 studying the phellem in the stem of the geranium, notice that 

 druses are occasionally found in some of the inner cork cells. If 

 your sections show a phellem four or five layers in thickness, you 

 will find that the tangential (and to some extent the radial walls) 

 are somewhat wavy or irregular. This condition, which is very 

 commonly found in corky tissue, results from the constant pres- 

 sure of the successively developing and enlarging cork cells upon 

 tlie older cells of the phellem. Ti/pical phelloderm, Jiowever, is 

 rarely developed in the young stem of the geranium. 



(c) Directly inside the innermost layer of cortical cells occurs 

 the stele, the outer region of which is indicated by the pericycle 

 which in the geranium stem consists of an outer continuous ring 

 of thick-walled pericyclic fibers (elongate tightly joined cells, as 

 seen in longitudinal section, with characteristic slit-like spirally- 

 arranged pits) followed by several layers of small thin-walled 

 parenchyma cells. The vascular tissue of the stele lies directly 

 within the pericycle and consists of a ring of typical collateral 

 bundles (i.e., bundles in which the phloem lies radially external to 

 the xylem) which are separated from one another, at least in 

 certain regions, by strips or bands of parenchymatous tissue 

 which because of their direct connection with the pith (medulla) 

 may be termed medullary rays. 



