THE VASCULAR CRYPTOGAMS 85 



above. Each of the ridges corresponds in position to one 

 of the vascular bundles (see Fig. 39). 



The Equiseta are characterised by a great development 

 of intercellular spaces, which have a very definite 

 arrangement. There is usually a ring of large spaces in 

 the cortex, and these cortical cavities are alternate in 

 position with the bundles, and thus lie opposite the 

 depressions or furrows of the external surface. For 

 this reason they bear the name of vallecular cavities. 

 Another ring of intercellular canals accompany the 

 bundles, one on the inner side of each ; these lie 

 opposite the ridges of the stem, and are consequently 

 called the carinal cavities. We shall see presently how 

 they arise. These canals are interrupted at the nodes. 

 Lastly, the whole interior of the pith of the internodes 

 often becomes hollow, leaving only a persistent dia- 

 phragm at each node. This almost always happens 

 in the main aerial stems ; but in the finer aerial 

 branches (see Fig. 39) and in the rhizome the pith often 

 remains solid, as is the case in E. arvense. The 

 intercellular spaces do not all fulfil the same function ; 

 the carinal cavities and the central cavity, if present, 

 usually contain water, while the vallecular cavities are 

 always full of air. 



In E. arvense and some other species the central 

 cylinder is well defined, a common endodermis sur- 

 rounding the whole ring of vascular bundles on their 

 outer side. In other species, however, there is a 

 separate endodermis round each individual bundle, as in 

 E. limosum (see Fig. 40). 



In others again there is an intermediate state of 

 things, for a common endodermis is present inside the 

 ring of bundles, as well as outside them (E. variegatum). 



