110 THE STEM 



111 view of the obvious variation in tlie size and degree of 

 development of the vascular bundles in any given section, the 

 following description is only general in nature ; and all essential 

 variations and details must be indivdually interpreted. 



A well-developed vascular bundle in the geranium stem is 

 somewhat wedge-shaped or triangular in cross-section and con- 

 sists of an external region of phloem tissue separated from the 

 internal region of xylem tissue by the camhial zone in which cell 

 divisions occur predominately in the tangential plane. 



Beginning with the phloem tissue of the bundle first of all, you 

 should be able to find the somewhat crushed primary phloem lying 

 directly against the innermost cells of the pericycle; structurally, 

 primary phloem in the geranium stem appears to consist of small 

 sieve-tubes, companion cells and phloem parenchyma. Lying 

 directly inside of the primary phloem occurs the secondary 

 phloem which is composed of (1) sieve-tubes, rather large poly- 

 gonal cells, apparently devoid of contents; (2) companion cells, 

 extremely small, more or less triangular cells (in cross-section 

 of course) closely joined to the sieve-tubes and usually containing 

 a definite nucleated protoplast; and (3) large thin-walled 

 parenchyma cells. 



Separated from the secondary phloem by the "cambial zone" 

 occurs the xylem tissue of the vascular bundle. In all of the 

 larger bundles, the xylem is of two kinds, viz.: (1) secondary 

 xylem, an external layer of thick-walled isndiametric, tightly 

 joined cells which are arranged in more or less definite radial 

 rows because of their origin from the fascicular cambium: and 

 (2) the primary xylem, an internal group of ratlier large, more 

 or less polygonal cells, irregularly arranged and imbedded among 

 isodiametric thin-walled parencliyma cells. The smallest cells 

 of the primary xylem (wliich has differentiated centrifugally 

 from the procambial strand) are found nearest the inner edge 

 of the vascular bundle and represent the first formed elements of 

 the protoxylem. Notice particularly that in many protoxylem 

 cells, portions of the spiral thickening have been torn from the 

 wall in the process of sectioning; in other cells, a ]neee of the 

 spiral band may be seen projecting into the lumen of the cell. 



