22 
Review. 
sionally explained in terms of the normal root-structure. On this 
view it may be suggested that the vascular arcs, etc. should be 
called “ meristeles ” and that a certain part (with indeterminate 
limits) of the ground-tissue may represent the pith of the distal part 
of the root, although continuous with the rest of the ground-tissue. 
It is now generally held that in some cases the endodermis may be 
differentiated from various tissues, e.g. in Strasburger’s examples 
in the genus Equisetum, where it appears that in “ astelic ” forms 
the single ring of endodermis (shown in the monostelic species) has 
become interrupted between the vascular bundles, and new arcs of 
endodermis have been differentiated from the tissue adjacent to the 
inner and lateral surfaces of each bundle so as to unite with the 
external tangential strips of endodermis, thus forming complete 
sheaths. Hence the endodermis does not form a dependable 
morphological limit, but its local absence may have some signi¬ 
ficance. It is usually absent on the inner faces of the vascular 
arcs in the base of the root, and this would agree well with the 
theory of the breaking up of a normal root-cylinder, for where it is 
now absent would be just where the vascular tissue originally 
adjoined the pith. 
Whatever view one takes as to the morphology of the tissues 
in the bases of these roots, it has no particular bearing on the 
structure of roots in general. 
There islittledoubtthatinquestions of the morphology of tissues, 
the comparative method is the most valuable. The careful com¬ 
parison of the mature structure of corresponding parts in a series 
of related species, where the differences are slight, will often 
enable one to follow the steps in the modification of a tissue, that 
is, in the extent of this tissue and in the nature of its elements. It 
is probable that one will have to appeal to data of this kind for the 
confirmation of wider generalisations belonging to the same subject. 
Hence it must be regretted that greater prominence was not given 
to this method in Dr. Drabble’s paper. From this point of view 
Corypha umbraculifera, which bears two kinds of roots, should 
certainly have been treated more fully. 
In the roots of palms the author includes the sclerenchyma, 
accompanying the vascular tissue, with the latter, and only speaks 
of “ pith” 1 when there is a central mass of parenchyma. By 
1 Dr. Drabble discards pith as a “ separate morphological entity.” 
This phrase certainly appears inappropriate, but it is quite 
justifiable to use the word pith for that part of the paren¬ 
chyma or sclerenchyma of the stele, which lies in a definite 
position, viz. internal to the vascular tissue. 
