THE PTERIDOPHYTA : LYCOPSIDA, ETC. 623 



bundle in the stem supplies a leaf. In Eqiiisetum, however, only the protoxylem 

 passes into the leaves, the metaxylem is entirely cauline, which means that 

 it belongs to the stem alone and has no connection with the leaves. 



There has evidently been considerable reduction in the evolution of this 

 type of bundle, which is to be expected if Eqiiisetum is descended from the 

 tree-like Calamites. This is borne out by the expansion of the metaxylems 

 at the nodes into a solid mass which unites with the protoxylems and obliterates 

 the carinal canals, thus forming a continuous siphonostele. The tracheids 

 of these nodal steles are very short and fat and are more for the storage than 

 for the passage of water. Physiologically this water storage tissue is associated 

 with the development at each node of a whorl of branches which obtain their 

 supplies from it. Sections at these levels, however, are quite suggestive of 

 the ordinary structure of the Calamite stem, for the xylem masses form a 

 continuous woody ring. 



Branch traces, which alternate with the leaf traces, are joined immediately 

 to the nodal metaxylem. The two metaxylem strands of each internodal 

 bundle spring directly from the nodal xylem, but the protoxylems are supplied 

 by branches which arise from the protoxylems of the internode below. Two 

 branches come from each protoxylem strand and, diverging to right and left, 

 pass across the inner side of the nodal xylem, and join with the corresponding 

 branches from the neighbouring bundles to form the protoxylem of the 

 bundles above. This rather complicated structure may be understood by 

 reference to Fig. 633. 



The whole structure is best interpreted as a widely perforated solenostele, 

 in which the perforations, separating the " vascular bundles," run the whole 

 length of each internode. They have been looked upon as branch-gaps, but 

 they show no constant relationship to the development of branches. It is 

 not improbable that it has been developed from an ancestral form with a 

 solid central xylem or protostele. 



The solenostelic interpretation is supported by the occurrence, at the 

 nodes only, of a second endodermis internal to the ring of vascular bundles. 

 From the above description of the stem we may note especially the follow- 

 ing outstanding points ; the large intercellular spaces which are associated 

 with the assimilatory tissue of the stem ; the small development of the xylem 

 and phloem, which becomes particularly marked in the marsh and aquatic 

 species of the genus, and the strong development of sclerenchyma at the 

 outside of the stem, which replaces the xylem as a means of mechanical 

 support. 



The apex of the stem is conical, with a large pyramidal apical cell with 

 three sides, the divisions of which follow in very regular order. The youngest 

 internodes are very short so that the sheaths of leaves overtop the apex like 

 a bud. The maturing internodes, however, expand so rapidly that the soft 

 tissues are torn apart, thus forming the three series of intercellular cavities. 



Branches are only formed on the green stems, not on the fertile ones. 

 The anatomical structure of the lateral branches differs only quantitatively, 

 not qualitatively, from that of the main axis. The number of leaves at each 



