Conducting Tissue-System in Bryophytct . 13 
leaf of Diplophyllum albicans. Our attention was called to 
this by Schiffner’s Fig. 64 H, p. IJ3. The cells of the ‘rib’ 
are oblong, and at the upper end show a tendency to spread 
out fan-wise and come into contact with a number of the 
ordinary square cells of the leaf (Fig. 11). The elongated 
cells contain less chlorophyll than the square ones, and their 
end-walls, as well as here and there the side ones, are 
irregularly pitted. Below, the cells of the rib are continuous 
with the similarly elongated cells of the stem. The tissue of 
the stem is quite homogeneous, showing no trace of a special 
conducting strand. 
The difference between such a case as this and the strands 
we have been hitherto considering will be sufficiently obvious. 
The strand in question is no doubt to be considered as 
rudimentary ‘ Leitparenchym,’ the forerunner, as Haberlandt 
has shown, of the leptom of the higher forms. Instead of 
a localization of the regions of absorption we have here a 
localization of the regions of assimilation, and a special need 
for ready removal of the products of assimilation owing to 
the square shape and thick walls of the assimilative cells 
which form the bulk of the leaf. 
II. The Histology of the Rhizome and 
Aerial Stem in Polytrichum. 
Although W. P. Schimper 1 , Fr. Unger 2 , and P. G. Lorentz 3 
laid the foundations of our knowledge of the anatomical 
structure of the Mosses, Haberlandt, in his admirable work, 
Beitrage zur Anatomie und Physiologic der Laubmoose 
(Pringsh. Jahrb., 1886), was the first to give a satisfactory 
detailed account of the principal types of tissue-structure, and 
to explain their physiology, not only by acute reasoning from 
the anatomical data, but also to some extent by direct 
1 Recherches anatomiques et morph ologiques sur les Mousses. Strassbourg, 
1848. 
2 Beitrage zur Physiologie der Pflanzen, vii, 1801. 
3 Grundlinien zu einer vergleichetiden Anatomie der Laubmoose. Pringsh. 
Jahrb., vi, 1867. 
