298 



BOTANY 



teeth soon become very evident, and the margin of the young sheath has a scal- 

 loped outline. The number of the primary teeth may be subsequently increased 

 by dichotomy of the apex. Occupying the axis of each tooth is a strand of elon- 

 gated cells, which develops into a vascular bundle bending downward into the 

 stem, where it joins a zone of narrow cells (procambium) immediately outside 

 the pith, and in the procambial zone the stem-bundles are later differentiated. 

 The first permanent elements to appear in the bundles are several small spiral 

 tracheids which arise upon its inner side. 



All of the tissue elements in the nodes remain short, and a certain amount of 

 active tissue (cambium) has been shown to persist in E. telmateia, and possibly 

 is present in some other large species, so that a limited increase in the diameter 

 of the stem is possible. 



The bundles run downward through the internodes and divide into two equal 

 parts at the node. Each branch joins a similar one for the neighboring bundle, 

 so that in any intemode a bundle is composed of the fusion of two branches for 



B. 



FIG. 263.Equisetum telmateia. A, section of a strong vegetative bud (x 30); 

 k, lateral bud. B, stem-apex (X 200). 



separate bundles of the intemode upon it, and the bundles in succeeding inter- 

 nodes alternate with each other. 



The structure of the completed bundle is collateral, with the xylem inward, the 

 phloem outward, like that of the Seed-plants, especially suggesting that of many 

 Monocotyledons. The primary tracheids are usually completely destroyed by 

 the development of an air-chamber on the inner side of the bundle (Fig. 264). 

 A group of secondary tracheids is formed on each side of this, but the other 

 xylem-elements remain unchanged. The outer part of the bundle, the phloem, 

 contains sieve-tubes not unlike those of the Ferns, and there may also be present 

 thick-walled fibrous cells. 



Outside of the ring of vascular bundles there is present in E. telmateia a con- 

 tinuous endodermis (Fig. 264), and in other species e.g. E. hiemale there may 

 also be present a second inner endodermis. Less frequently each bundle has a 

 complete endodermis about it (E. limosurri). 



The green assimilative tissue is confined to the stems, especially to the slender 

 secondary branches. In the main shoots the green tissue, in transverse section, 



