Development, and Structure of the Vegetable Cell. 275 
fluid containing tannic acid is diffused. These lacteal vessels 
resemble the ramified fibrous cells of the bark, in sending off 
branches into the intercellular spaces, the membranes of which, 
from their extreme tenuity, may be readily overlooked, and thus 
give rise to the inference that the lacteal fluid circulates within 
the intercellular spaces themselves. I believe I have proved in 
that memoir that the lacteal vessels do actually possess walls of 
their own ; they are therefore, with reference to their mode of 
origin, analogous organs to the fibrous cells of the bark. 
The processes which go on in the formation of the so-called 
callus on the lower part inserted into moist earth are precisely 
similar to this formation of periderm ; and this was particularly 
the case in cuttings of Philodendron when these were terminal 
shoots, and had continued to develope themselves after their 
separation from the lower part of the stem. 
Under these cireumstances also, a layer of cork is first pro- 
duced on the cut surface (which, owing to its being implanted 
in the moist earth, scarcely undergoes any desiccation), to pro- 
tect the living tissues from the access of air. However, the 
cork-lamina does not acquire the same proportion as in the pro- 
cess of cicatrization in the air, nor does it penetrate so deeply 
within the plant-tissue near the epidermis and vascular bundles; 
and after the peridermic layer has attained a certain thickness, 
the cells formed deeper within the tissue are not cork-cells, but, 
like the cambium-cells of the apex of the axis, become converted 
into the most various histological elements. 
Two cell-nuclei are frequently met with in one mother cell, 
each containing either two nucleolar corpuscles or two vesicles 
of a larger size and having each two nuclear cells, whilst no 
cell-nucleus belonging to the cell-system of the mother cell is 
contemporaneous with them. 
Criiger, who first fully described (Botanische Zeitung, 1860, 
p- 869) the changes in the cell-tissue during the formation of 
callus, supposed that no new formation takes place in the cut 
vessels. This, however, is an error ascribable without doubt to 
the great tenuity of the walls of the endogenous cells, the 
_ appearance of which is a counterpart of what has been already 
described and figured (Pl. V. figs. 3-5), except that these cells 
are still more hyaline and transparent, inasmuch as they are 
defended from the contact of the air, and consequently not ren- 
dered suberous (verkorkt). ; 
I have found the vessels contiguous to the callus filled with 
new cells, not only in endogenous but also in exogenous plants 
—for example, besides Philodendron, in Zingiber, Dracena, Zamia, 
Cycas, Ficus, Gesneria, &c. In all cases the fully developed 
vessels have a much greater tendency to generate new cells in 
