143 
The Stelar Theory. 
into the central cylinder.” In no other sense than this can homology 
possess any meaning whatsoever. But to this origin of the pith 
our author will not subscribe. “Doubtless,” he continues, “the 
pith has arisen phylogenetically from stelar cells—at least if the 
solid vascular strand constitutes the phylogenetically oldest stage—; 
there can therefore in that process be no more question of intrusion 
than there exists at present in the case of the seedling-plant.” 
This is supported by the detailed facts of development of the pith 
described by Farmer and Hill in Angiopteris. 
Then with regard to the question as to the value to be attached 
to the presence of the endodermis in the form of either a protective 
or a starch-bearing sheath, our author holds that Jeffrey, Van 
Tieghem, and others have greatly exaggerated this. The endodermis 
may, from its well-known histological characters, be generally 
recognised as the morphologically inner layer of the cortex; these 
characters are therefore of importance for the determination of 
morphological relationships. But it by no means follows from this 
that every layer which is histologically differentiated as a “protective 
sheath” is necessarily an endodermis in this sense. The idea ot 
Farmer and Hill that there is “a strained and artifical criterion as 
to the boundary of the stele” has arisen solely from the fact that 
the “protective sheath” (“Schutzscheide”) which is a histological 
diffentiation, has been confused with the morphological conception 
of the endodermis. 
Erroneous views have also been put forward by Pitard, with 
regard to the morphological value of the pericycle. The author 
observed this layer in the older plant-organs becoming torn, and 
cells of other tissues intruded between the pericycle cells ; but the 
conclusion he arrives at—that when in this way the activity of the 
tissue ceases, it is no longer to be regarded as a pericycle, is as 
absurd and erroneous as it would be to conclude from the fact that 
epidermis and cortex are often cut away by an internal periderm 
and are thus actually absent, that these tissues possess no morpho¬ 
logical value as such. 
Our author next discusses and disposes of the views of Van 
Tieghem with regard to “ Polystely ” and “ Astely ” as applied to 
the stem, showing that these are merely modifications of the 
fundamental monostelic condition. But in leaves it is different; 
although in many of these monostely is clearly present in the 
petiole, yet from the fact that in the lamina the bundles always 
exhibit an independent course and a distinct sheath, and the absence 
of any data to show the derivation of the leaves of modern plants 
from monostelic leaves, it appears best to conclude that the 
structure of the lamina has always been different from that of the 
stems and petioles with their rounded contour. True astely as 
contrasted with monostely, exists therefore in the leaf-lamina ; while 
Strasburger’s term “ schizostely ” may be applied to that variety of 
monostely occurring in the stem in which a “ protective sheath ” 
is present around each bundle. Yet the author admits that astely 
may possibly be a modification of monostely, and if so must be of 
greater importance than schizostely because more widely distri¬ 
buted and more distinctively modified; he admits the presence in 
some plants of a schizostelic structure in the petiole or large veins 
forming a transition from the monostelic condition of the stem 
