3i8 



THE PRIMARY STEM 



lateral and horizontal walls do not always (as in the 

 root) possess a central band of cutin (see p. 293), though 



Fig. 52. — Structure of the primary stem. A, diagram of cross-section 

 of stem showing vascular bundles arranged in a cylinder; X, 

 xylem ; px., protoxylem ; P, phloem. B, single bundle in cross- 

 section ; cor., cortex; end., endodermis ; ■per., pericycle (fibrous 

 opposite bundle). C, diagram of segment of stem seen obliquely 

 from above. The cortex and conjunctive tissue of the cylinder are 

 supposed to be transparent, the epidermis and endodermis opaque. 

 D, diagram of the course of the leaf trace and stem bundles 

 at a node bearing two opposite leaves in Clematis. The bundles 

 towards the observer are black, those away from the observer 

 shaded. Three bundles enter the stem from each leaf. The 

 laterals {/) at once join bundles from the intemode above (two 

 of which fork), the central bundle (c) of each trace continues alone 

 into the intemode below (joining with others at a lower node). 



sometimes they do. The stem endodermis often differs, 

 however, in other ways from the cells of the cortex 



