On Secondary Thickening in Pteridophyta. 209 
(ii.) Appearance. The secondary tracheides are often of 
somewhat slow development, and in many cases there may be seen 
immature elements with their walls partially lignified and still 
retaining living contents. 
(iii.) Relation to adjacent tissues. There may be seen a more 
or less definite seriation, the elements appearing in radial rows. 
This character must, however, be used with caution, for the cells 
of the primary tissues may sometimes exhibit a radial arrangement. 
OPHIOGLOSSACEzE. 
Literature. 
Boodle.—On Some Points in the Anatomy of the Ophioglosseae. 
Ann. Bot., Vol. XIII., 1899. 
Russow, — Vergl. Untersuchungen. Mem.de l’Acad. imp. des 
Sciences de St. Petersbourg. Ser 7, t. XIX., 1872. 
Botrychium lunaria. 
Stem. The presence of cambium, and its activity in the rhizome 
of this plant, is so well-known that nothing more than the briefest 
description is necessary. It is sufficient to observe that the 
cambium occurs in a normal position between the primary wood 
and the phloem, and constantly adds new elements to these two 
tissues. The process is thus of a type characteristic of a normal 
Dicotyledon or Gymnosperm. 
Root. Boodle has shown that there is a considerable addition 
of secondary tracheides in the bases of the roots, more especially 
in those regions which are embedded in the cortex of the rhizome. 
The characters of this later formation of new tissue is of the same 
nature as that of Ophioglossum described below. 
Orhioglossum vulgatum. 
Stem. An addition of secondary tracheides takes place on the 
outer margin of the primary wood of the older parts of the rhizome, 
and it is to be observed that there is no definite cambium. The 
later-formed elements may be recognized by their reactions with 
certain stains (see Boodle, loc cit.) 
Root. The phenomenon in question is similar to that which 
obtains in the rhizome, but the addition of tracheides takes place on 
all sides of the xylem, and not merely on the outer margin as 
in the stem. As before, there is no cambium ; there may, however, 
sometimes be made out a radial arrangement of the new elements; 
but, as has been pointed out above, this feature is not to be 
absolutely relied upon in determining which elements are primary 
and which are of secondary origin. 
