82 Harshbererer. — Maize 



thicker cell walls. The bundles of the stem run parallel 

 along the internodes without intercrossing. A typical 

 central bundle is chosen in describing the closed collateral 

 bundle of maize, because at the surface the bundle elements 

 are reduced to a minimum, and because here a union 

 frequently occurs between two or three bundles. The sur- 

 rounding layers of the bundle consist of strongly thickened 

 sclerenchyma, the external and internal cells having thicker 

 walls than those on the lateral faces. A large intercellular 

 space surrounded by comparatively thin-walled cells attracts 

 the attention. An annular tracheid outside, and a spiral 

 tracheid between two large reticulated vessels are elements 

 forming the wood, xylem, hadrom or vascular portion of the 

 bundle. External to it we have the bast, phloem, leptom 

 or sieve portion of the bundle. The phloem portion colors 

 a bright violet with chlor-iodide of zinc. The first elements 

 of importance are sieve tubes with "companion" nourishing 

 cells. The proto-phloem {Cribral primaneti), or actively 

 growing leptom, is found in this area. 



The bundles at the periphery of the stem are crowded 

 together, so that the large intercellular space disappears ; the 

 phloem portion is also reduced. The bundle sheath, which 

 alone remains to any extent, is continuous with a layer of 

 thick-walled cells, called by Strasburger the hypoderm. The 

 hypoderm and bundle sheath function as protective and 

 strengthening tissues, and are elements in the mechanical 

 system of the plant. 1 The separate cells of the hypoderm 

 have been called stereids, and the whole tissue taken together 

 has been called a stereome. 



The epidermal cells in a radial longitudinal section are 

 longer than broad. The ordinary parenchyma cells are round, 

 or nearly so. The cells of the bundle sheath are long, with 

 contracted lumen. The intercellular passage follows the 

 length of the bundle without a break. The annular tracheid at 

 the inner border of the space is one of the most characteristic 

 elements of the stem. The large dotted ducts to the right 

 and left are shown in the section figured. Between the two 



i Pfitzer, Pringsheim's Jahrb., Bd. vm. 



