102 THE STEM 



the complications in vascular anatonn- resulting from the devel- 

 opment of leaves and buds, offer a series of problems which are 

 bej'ond the scope of the present book. In general, however, it 

 may be stated that in gymnosperms and dicotyledons, the cylin- 

 der of vascular tissue is interrupted at or near the node by the 

 development (early in ontogeny ) of leaf gaps, which are paren- 

 chymatous areas in the siphonostele situated above the point of 

 divergence of the leaf traces. Depending upon the nature of the 

 foliar structure as well as upon the plant, the anatomy of the 

 node is described as unilaeunar (one gap), trUacunar (three 

 gaps) or multilacunar (more than three gaps). There appears 

 to be some evidence that in angiosperms the trilacunar node is 

 the primitive condition. (For further details cf. Eames and 

 MacDaniels, pp. 114-120.) The development of the axillary bud 

 results in additional complications in the vascular anatomy of 

 the node. The earliest vascular bundles of the bud are known 

 as branch traces and their "divergence" from the main axis 

 likewise is associated with parenchymatous areas in the stele 

 which are termed branch gaps. Wide application of the prin- 

 cipals of nodal anatomy has been made in the study of the vas- 

 cular anatomy of the flower (cf. Eames, 1931, and Wilson and 

 Just, 1939). 



1. The primary structure of the stem. In man}' of the k)wer 

 vascular plants and in certain herbaceous angiosperms (particu- 

 larly monocotyledons), all the stem tissues are primary, i.e., they 

 originate directly from the progressive differentiation of cells 

 derived from the apical meristem of the shoot. Despite the great 

 variation in the kinds and patterns of tissues, a common plan of 

 primary structural organization is found in the stems of most 

 gymnosperms and angiosperms. This consists in the existence of 

 three more or less well-demarcated zones or regions which now' 

 may be described briefly as follows : 



{a) The epidermis. Stems possess a well-defined epidermis in 

 which stomata and various types of trichomes may be present in 

 addition to typical epidermal cells (cf. Exercise V). 



(b) The cortex. Beneath the e]ndermis of stems is found a 

 cylindrical zone, variable in its radinl dimension and in the Idnds 

 of cells which occur. This region is the cortex and in the simplest 



