252 



PLANT MORPHOLOGY 



form a sheath around the node. The number of leaves at a node corre- 

 sponds to the number of ridges on the stem, each leaf standing directly 

 above a ridge of the intcrnode directly below it. The stem branches are 

 not axillary but arise at the node alternately with the leaf primordia and 

 at the same level, later breaking through the united leaf bases. Thus the 

 number of branches at a node usually equals the number of leaves. 

 Growth of the root and stem takes place by means of a tetrahedral apical 



cell that cuts off three rows of lateral 

 segments with striking regularity (Fig. 



209). 



Vascular Anatomy. The stem of 

 Equisetum is characterized by a much- 

 reduced vascular system. The greatest 

 development of xylem occurs at the 

 nodes, where it forms a transverse 

 band. From here a leaf trace goes to 

 each leaf, forming a single median vein. 

 A cross section through an internode 

 shows an extensive cortex bounded ex- 

 ternally by an epidermis, a circle of 

 small, isolated, vascular bundles sepa- 

 rated from one another by broad bands 

 of parenchyma, and a hollow pith (Fig. 

 210^1). The cortex is pecuhar in hav- 

 ing a ring of large air spaces, called 

 vallecular canals, one of which lies 

 beneath each furrow present on the outer surface of the stem. 



The epidermis has thick, strongly sihcified cell walls. Underlying it is 

 a band of sclerenchyma projecting inward beneath the ridges and some- 

 times not continuous across the grooves. Green tissue occupies most of 

 the cortical region, occurring largely or entirely beneath the furrows. 

 Stomata, communicating with the green tissue, are situated in the 

 grooves. Their guard cells are peculiar in that each Ues inside and next 

 to a subsidiary cell, so that they seem to be double. 



Internal to the vallecular canals and alternating with them, and so 

 lying beneath the ridges, are the smaller carinal canals, one of which 

 belongs to each vascular bundle. The carinal canals mark the position 

 of the protoxylem, the disorganization of which results in their formation. 

 The metaxylem, which is greatly reduced in amount, lies along the sides 

 of and external to the carinal canals (Fig. 210B). It develops centrif- 

 ugally, a condition designated as endarch. A small group of phloem 

 elements, consisting of sieve tubes and parenchyma, are present. The 

 phloem has a collateral relation to the xylem. No secondary tissues are 



P"iG. 209. Median longitudinal sec- 

 tion through the stem tip of Equisetum 

 arvense, showing apical cell and its 

 derivatives, X200. 



