some Species of Gleichenia. 429 
lower part of the petiole, and, secondly, that Eugleichenia , in which this 
remnant seems to have entirely disappeared, is less primitive than Mertensia , 
in which it still remains. 
Turning now to the external morphology, we find that in Mertensia the 
pinnules are usually long (10-40 mm.), while in Eugleichenia they are rounded 
and short (1-2 mm.). This is consistent with xerophytic reduction having 
taken place in Eugleichenia , and thus agrees well with the anatomical results. 
For the reasons given above, Eugleichenia is to be regarded as consist- 
ing of a series of reduced forms, and as they appear to have been derived 
from forms of the Mertensia- type, the most primitive living species of 
Gleichenia should be sought for in the subgenus Mertensia , and G. flabellata 
may be taken as one of the most primitive species (Tansley, ’ 07 , p. 142). 
It has a simple protostele with a horseshoe-shaped petiolar bundle. 1 It is 
interesting that, in soral characters also, this species appears to be primitive. 
On account of its possessing a pre-eminently large number of spores in the 
sporangium, and a radiate-uniseriate type of sorus in its most regular form. 
Bower (’ 08 , p. 559) regards this species as probably the most primitive of 
those which he examined. 
The existence, however, of solenostelic structure in G. pectinata raises 
a doubt as to whether the protostelic forms may not have been derived from 
solenostelic ancestors by reduction ; the nodal islands of the simpler forms 
might then be regarded as remnants of the original central core of the 
solenostele. But a careful examination of the structure of G. pectinata 
shows that there is no sound basis for this view, for in this species the same 
type of nodal island is present, in addition to the central solenostelic com- 
plications (Tansley, ’ 07 , p. 142). Thus, until further evidence can be 
obtained, it seems justifiable to consider protostelic structure as primitive 
for the genus Gleichenia . 2 
We may now consider the origin of the solenostelic structure of G. pecti- 
nata , deriving it from the protostelic type of structure found in Mertensia . 
Three possible courses of evolution for the solenostele may be suggested : — • 
1. By decurrence 3 from a leaf-trace, or 
1 a. From a branch of the rhizome. 
2. By decurrence 4 from a branch of the rhizome, the protostele having 
previously become medullated. 
3. Independently of decurrence, by a series of symmetrical changes in 
the rhizome. 
1. Decurrence from the leaf-trace of G. pectinata gives rise to a nodal 
1 For a description of the nodal structure see Boodle (’Ot), p. 723. 
2 It is perhaps possible, but under the circumstances not likely, that the protostelic forms might 
have been derived from the G. pectinata type by suppression of the tissues within the xylem-ring and 
retention of the nodal pocket. 
3 This term is used metaphorically to imply basipetal extension by transformation of tissue in 
a phylogenetic series. See Boodle (’ 03 ). 4 Or procurrence ; see below. 
