EQUISETUM. SPOROPHORE. 215 



c. Large Intercellular cavities, which alternate 

 in position with the ridges, and are thus opposite the 

 channels of the outer surface. 



iv. The cortex is limited internally by a single 

 sinuous layer consisting of cells in close contact with 

 one another ; this is the Bundle-sheath (see below) : 

 it forms a continuous and sinuous ring surrounding 



v. The Vascular bundles, which may be recog- 

 nised as oval groups of elements of smaller size than 

 those of the surrounding tissue : they alternate in 

 position with the intercellular cavities of the cortex, 

 and are thus opposite to the ridges which project 

 on the external surface. 



vi. The Pith, which lies centrally, consists of thin- 

 walled tissue, and is in great part obliterated by a large 

 central cavity. 



III. Before proceeding to the more minute study of 

 these several tissues, cut transverse sections through a 

 leaf-sheath : mount in glycerine, and examine with a 

 low power. It may be observed that the arrangement 

 of tissues is not unlike that of the peripheral tissues 

 of the internode. Note especially that as in the inter- 

 node, so also in the leaf-sheath, one vascular bundle 

 (here of small size and simple structure), is to be found 

 opposite each ridge. 



IV. Cut a series of rather thick transverse sections 

 through the nodal region : it will be best to select one 

 which bears no fully-developed, lateral branches. 

 Keep them all in their right order of succession, 

 mount, and compare them under a low power, starting 

 from such a section above the node, as will show an 

 arrangement of tissues typical of the internode, together 



