. Elementary botany. Botany. ORGAXIZATIOiV: PLANT TISSUES. 361 perennial plants continue to grow in diameter each year. The cambium in the open bundle forms new tissue each spring and summer, thus adding to the phloem on the outside and the xvlem on the inside. In the spring and early summer the large vessels in the .wlem predominate, ^^â hi]e in late summer wood fibers and small vessels predominate and this part of the wood is firmer. Since the vascular bundles in the stem form a circle in the cylinder, this difference in the size of the spring and late summer ^^â ood produces the "annua
. Elementary botany. Botany. ORGAXIZATIOiV: PLANT TISSUES. 361 perennial plants continue to grow in diameter each year. The cambium in the open bundle forms new tissue each spring and summer, thus adding to the phloem on the outside and the xvlem on the inside. In the spring and early summer the large vessels in the .wlem predominate, ^^â hi]e in late summer wood fibers and small vessels predominate and this part of the wood is firmer. Since the vascular bundles in the stem form a circle in the cylinder, this difference in the size of the spring and late summer ^^â ood produces the "annual" rings, so evident in the cross-section of a tree trunk. Branches originate at the surface in"\'olving epidermis, , and the bundles. In monorotyledonous plants (corn, palm, etc.) the bundles are not regu- larly arranged to form a hollow cylinder, but are irregularly situated through the stele. There is no meristem, or cambium, left between the .XA'lem and phloem portions of the bundle and the bundle is thus closed Cas in fig. 60), since it all passes over into permanent tissue. In most monocotyledons there is, therefore, practicallv no annual increase in diameter of the stem. 711. Ferns.âIn the ferns and most of the Pteridophytes an apical meri- stem tissue is wanting, its place being taken by a single apical cell from the several sides of which cells are successively cut off, though Isoetes and many species of Lycopodium have an apical meristem group. In most of the Pteridophytes also the bundles are concentric instead of col- lateral. Fig. 418 represents one of the bundles from the stem of the poh'pody fern. The xvlem is in the center, this. Sectio Fig. 410. tern (rhizome) of surrounded by the phloem, the phloem by p^^.^^ aquilma. , th,ck-«-alled the phloem sheath, and this in turn bv sclerenchyma; a, thin - n-alled . ' sclerenchyma; ^ar, parenchvma. the endodermis, giving a concentnc ar- rangement of the component tissues. .\ cross-section of the
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