PTERIDOPHYTA 



253 



formed. Generally a continuous endodermis surrounds the ring of vas- 

 cular bundles, but often an endodermis occurs both inside and outside the 

 bundles. Sometimes each bundle is enclosed by its own endodermis. 

 Although the vascular bundles are small and widely separated, because of 



outer 

 endodermis 



carina/ 

 canal 



inner 

 enaodermis 



mefaxylem 

 protoxylem 



Fig. 210. Stem structure of Equisetum hyemale var. intermedium. A, portion of cross 

 section of an internode, showing thick-walled epidermis with three stomata, sclerenchyma, 

 green tissue, two vallecular canals, and three vascular bundles, X 100; B, a single vascular 

 bundle, X250. 



their arrangement and the position of the xylem and phloem, the stem of 

 Equisetum is an ectophloic siphonostele. 



Sporangium. The strobili of Equisetum are solitary and terminal on 

 the main stem or sometimes on its branches. Generally they are borne on 

 ordinary green shoots, but in some species they occur on special shoots. 

 In Equisetum arvense, for example, there are two kinds of aerial shoots. 

 They arise from the same rhizome but at different times of the year. 



