THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



625 



node is reduced to three or four, with a corresponding reduction in the 

 number of bundles in the stele, and the assimilating tissue is increased, forming 

 three or four ridges along the internodes. The close relationship between the 

 main axis and the side axes is shown by the fact that in the early stages of its 

 development the main axis has a structure which is exactly like that of the 

 laterals which are formed later. Furthermore, in some examples, the apex 

 of the main stem is again reduced to this simple structure and forms a delicate 

 prolongation above the level of the highest laterals. 



The xylem system of Equisetum seems so inadequate for the maintenance 

 of the vigorous water flow which can be demonstrated experimentally, that 

 the suggestion has been made that water travels in the intercellular canals. 

 In spring it is not unusual to find all these canals full of water under pressure, 

 so that the suggestion of their conducting function, at least under some 

 conditions, is probably correct. 



Anatomy of the Leaf 



The leaves plav an unimportant part in the life of Equisetum, for they 

 cannot function efficiently as organs of assimilation and probably serve 

 chiefly to protect the lateral buds. 



The tips of the leaves are free, though their lower parts are fused laterally 

 to form the leaf sheath round the base of the internode. Each leaf contains a 

 single vascular bundle. 



The vascular bundles of the leaf sheath are simple and collateral in 

 structure and do not possess carinal canals. Each bundle is surrounded by 

 an endodermis. The outer tissues of the leaf sheath consist of narrow bands 

 of sclerenchyma, which pass up the leaf ridges, alternating with the strips of 

 assimilatory tissue associated with stomata. We thus have, on the sheath. 



Outer cortex with 

 Exdodermis 



Inner cortex 



Endodermis 

 Inner Endodermis 



Protoxylem 

 Metaxylem 

 Phloem 



Fig. 634. — Equisetum arvense. Transverse section 

 of the root. 



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