396 Farmer and Hill.— Arrangement and Structure 
embrace those cases in which the vascular tissues had passed 
out of the protostelic condition (in which a solid core of 
xylem occupies the axis of the stele), but he retained the 
inner endodermis as an essential constituent delimiting the 
inner region of the stele from the axile parenchyma, which he 
rightly, as we think, regards as not belonging to it. 
We regard this as a step in the right direction, but in our 
opinion Jeffrey has not gone far enough. The great advan- 
tage which the idea of the siphonostele possesses over that 
of the monostele for us, lies in the emphasis which it throws 
on the unity of the vascular tissue as a whole as opposed 
to the circumjacent ground-parenchyma, instead of merely 
laying stress on one particular arrangement or collocation 
of tissues which may or may not be preserved in its integrity 
according to the nature, firstly, of the foliar gap development, 
and secondly, of the internal differentiations which may occur 
in connexion with them. We prefer, as a preliminary step, to 
dissociate the stele altogether from the endodermis which 
is certainly not always present. For us it is a matter of 
absolutely subordinate importance whether there is an internal 
endodermis or not ; and, so far as we know, there is no in- 
herent or a priori need of postulating its theoretical existence 
when it cannot be objectively demonstrated. 
The inherent weakness of the position which relies on the 
existence of an internal endodermis, as forming an essential 
zone of demarcation of the stelar tissue from the medullary 
parenchyma, is rendered manifest by many test cases in which 
it may or may not be present. Thus in the young rhizome 
of Helminthostachys zeylanica 1 it does not occur, whilst it 
appears fitfully and irregularly within the steles of older plants. 
Again, the case of the genus Botrychium is of interest in 
this connexion. Van Tieghem 2 regarded the stem of B. 
lunaria (together with the other members of the Ophioglos- 
1 Farmer and Freeman, On the Structure and Affinities of Helminthostachys 
zeylanica . Annals of Botany, xiii. Cf. also W. H. Lang, Prothalli of Ophio- 
glossum pendulum and Helminthostachys zeylanica. Annals of Botany, xvi, p. 43. 
3 Van Tieghem, Remarques sur la structure de latige des Ophioglossees. Journ. 
de Botanique, t. iv. 
