THE ANATOMY OF THE SHOOT 63 



constitute the inner bundles of the meristele. The latter are usually 

 the median or larger bundles of the leaves, while the smaller, 

 marginal bundles are those which occupy the peripheral portion of 

 the stele. The more crowded distribution of bundles in the outer 

 zone of the vascular region is due to the fact that the bundles, 

 except for the peripheral ones, do not describe a perpendicular 

 course in the stem axis, but are differentiated in a radially oblique 

 direction until they finally anastomose at lower levels with bundles 

 of the outer zone. Since a single bundle may continue through 

 several internodes before being united with another one, the effect 

 of intercalary elongation of the axis is to straighten out the curva- 

 ture, so that each bundle apparently describes a perpendicular 

 course in the mature internode. The insertion and anastomosis of 

 bundles with those lower in the axis occurs chiefly at the nodes, 

 where a very complex nodal plate is formed consisting of a large 

 number of transversely and otherwise oriented bundles. 



The individual bundles which constitute the meristele vary in 

 type. The most common is the collateral or half-amphivasal 

 bundle, and less frequently a true amphivasal bundle is differenti- 

 ated. A third type is the amphicribral bundle which commonly 

 occurs in pteridophytes (Polypodium), but is rarely developed in 

 angiosperms. In the collateral bundle of the meristele, the vascu- 

 lar tissues are usually oriented so that the xylem is centrad to the 

 phloem; and, in amphivasal types, it more or less completely 

 encircles the phloem with outwardly projecting arms of metaxylem, 

 the development of the primary xylem being endarch. The 

 mechanical tissue associated with the bundles consists of fibers or 

 sclerotic cells which form a more or less complete ring around the 

 bundle (Zea and Triticum). In addition to this mechanical tissue, 

 the stem may have a zone of sclerenchyma immediately inside the 

 epidermis, which is several layers in thickness and consists of 

 small, compact, elongated elements. 



Ontogeny of the Stem Axis. — The apical meristem is pro- 

 tected from injury by the developing leaves and the bud scales when 

 present. An unelongated axis of this kind with its leaves is called a 

 bud, and its terminal portion consists of meristematic cells from 

 which lateral members are differentiated exogenously. (Fig. i6.) 

 The ontogeny of the stem axis is complicated by the manner in 

 which these lateral members are initiated and differentiated, since 

 the stelar organization of the stem depends upon the development 



