. Practical botany. Botany. THE STEM AND THE LEAF 49 of it are, as in the case (if the hollow flower stalk of the garden rhubarb and the dandelion. More frequently the stem is not hollow, but a large mass of very light spongy pith occupies the interior, as in corn, young twigs of elder (Fig. 34, A) and sumach, and in the entire stem of the sunflower. The stiffness of the young stem may lie due almost wholly to collenchyma, as in the balsam (Fig. 32) and the elder (Fig. 34, A), or it may depend largely on the presence of wood fibers and tracheids in the bundles, as in the sunflower (Fig. 30). S
. Practical botany. Botany. THE STEM AND THE LEAF 49 of it are, as in the case (if the hollow flower stalk of the garden rhubarb and the dandelion. More frequently the stem is not hollow, but a large mass of very light spongy pith occupies the interior, as in corn, young twigs of elder (Fig. 34, A) and sumach, and in the entire stem of the sunflower. The stiffness of the young stem may lie due almost wholly to collenchyma, as in the balsam (Fig. 32) and the elder (Fig. 34, A), or it may depend largely on the presence of wood fibers and tracheids in the bundles, as in the sunflower (Fig. 30). Sometimes collenchyma and fibers cooperate, as shown in the flower stalk of Eriimjunii (¥\g. 34, B). In the coll-. /- cart Fig. 3i. Arrangement of strengthening tissue A, B, in stems; C, in the root A, cross section of a yoimg elder twig; B, cross section of flower stalk of Eryn- gium; C, cross section of a small root; coll, collenchyma; cnrt, brittle cortex : all, tough central cylinder; /, fibrous cylinder around a central hollow portion ; p, pith; m, woody bundles surrounding the pith. After Strasburger case of dicotyledonous trees the stiffness of the trunk, resisting the severest storms, is mainly due to the enormous number of tracheids and fibers in the wood of the annual cylinders. The stems of woody climbers need to hr at once tough and flexible. Many such vines ha^'e, while young, the structure shown in the cross section of Dutclmian's-pipe (Fig. 2',}), with the bundles arranged in a discontinuous series around the central pith and not united into a cylinder. This makes the stem flexible in the same way that a wire cable is more flex- ible than a solid metal rod. Roots (except prop roots) do not need to possess much stiff- ness ; it is necessary for them to be tough to resist lengthwise. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not pe
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