THE ANATOMY OF THE SHOOT 



6i 



there is a peripheral zone of collenchyma which may form a con- 

 tinuous, several-layered cylinder immediately adjacent to the 

 epidermis. In some cases it may be restricted to limited regions, 

 as in angular stems where strands of collenchyma form reinforcing 

 ridges and add greatly to the mechanical strength and pliability of 

 the plant as a whole. 



The epidermis constitutes the outer protective layer of the axis 

 and is typically uniseriate. The cells of this tissue are compactly 

 organized into a continuous layer which is uninterrupted except 

 for stomatal openings. The outer and radial walls are usually 



Fig. 14. Transection of a portion of a stem of Triticum showing the arrangement of 

 the bundles, the distribution of the chlorenchyma and mechanical tissue, and the hollow 

 center. 



thicker than the inner ones, being cutinized or suberized; and 

 they may be further water-proofed by the deposition of a non-cellu- 

 lar cuticle. Epidermal hairs, prickles, thorns, and other emer- 

 gences may occur as noted in Chapter I. 



The Herbaceous Monocotyledonous Stem. — The stem of the 

 herbaceous monocotyledon differs from that of the dicotyledon 

 chiefly in respect to the components of the vascular bundles, which 

 in most cases do not possess a cambium. The arrangement of the 

 bundles may conform to the dictyostele type but meristeles are 

 common. In Triticum, the stem bundles comprise a dictyostele 

 with a central medullary region which may be hollow; while, in 

 Zea, there is a meristele and the bundles are scattered through the 

 fundamental parenchyma of the axis. (Figs. ^4, 2.5.) 



