Danctea and other Marattiaceae. 519 
the new theoretical interpretations. Strasburger’s term phloeo - 
terma and Jeffrey’s siphonostele may be cited as examples of 
these. The original, as well as the later terms, have in some 
respects the unfortunate character that they are not merely 
descriptive, and therefore cannot be used in many cases where 
they would be very useful if they did not commit the user to 
expressing homologies which he did not wish to imply. It 
seems desirable therefore, in this connexion, that certain names 
should be found, which denote definite structures without 
necessarily involving any idea of homology. The advantages 
of some such way out of the difficulty seem to counterbalance 
the disadvantages of a further multiplication of terms. 
In the terminology to be used in the present paper the 
word c stele ’ and several of its compounds will be employed 
in their original sense, in most cases, but several others 
have been devised to fill certain apparent gaps. 
The Dicotyledons are looked upon as the highest expression 
of plant life, and it therefore seems appropriate to apply the 
term eustele to their well-known type of vascular cylinder, 
i.e. a ring of collateral vascular bundles surrounding a pith 
and limited on the outside by the pericycle. Eustele is there- 
fore equivalent to Van Tieghem’s ‘monostele’ in one of its 
applications. This definition is adopted so that E. C. Jeffrey’s 
term of siphonostele may not be interfered with, because from 
his point of view the pith is extrastelar. In most roots the 
arrangement of the xylem and phloem is ‘ radial,’ hence the 
term actinostele seems appropriate. Its application, however, 
would not be confined to roots (cf. stem of Lycopodium ). 
Certain supposed primitive steles have been called protosteles , 
but at least two distinct kinds are included under this name, 
viz. what is here called an actinostele and also another simpler 
type in which a ring of phloem surrounds a mass of central 
tracheids (e.g. most Hymenophyllaceae). On account of its 
structural simplicity haplostele will be used for the latter type, 
and, if a central mass of parenchyma is present, the structure 
can be spoken of as a medullated haplostele (e. g. most Lepido- 
dendreae). The adoption of this word leaves protostele and 
