418 Prof. H. Karsten on the Formation, 
until they assume polyhedral forms, which occupy the central 
space of the joint-cells—are seen (as in fig. 31 x) pushing for- 
wards beneath the cells filled with chlorophyll which occur on 
the surface. 
During the period of the multiplication of the joint-cells, 
either the larger of these endogenous cells filled with secretory 
matter are, as it would seem, entirely absent, or else those are 
chiefly present which enclose in their readily soluble membranes 
the smaller ones containing chlorophyll and starch. These can 
then only be recognized in the vicinity of large endogenous cells 
which are no doubt developed into new joint-cells. 
The relative proportion in which these two forms of endo- 
genous cells are present, and their consequent position in the 
joint-cell of the Confervee, cause the wide range in form which 
prevails in these plants. 
As the mode of development and distribution of the endo- 
genous cells is dependent upon the nutrition of the plant, and 
consequently upon the chemical composition of the fluid in which 
it grows, the cultivation of Cladophora glomerata will furnish 
important materials for a revision of the systematic value of 
those forms. 
The great tendency to the diffusion of fluid which the trans- 
parent cells exhibit causes these cells, when the uninjured plant 
is placed in a solution of tannic acid, to expand greatly and to 
become visible on the surface after some time has been allowed 
for the reagent to act. This reagent will frequently bring to 
light the presence of cells within joint-cells, even although be- 
fore quite invisible, the whole substance looking like an unor- 
ganized mucilage. This is well seen in the case of the colourless 
contents of the joint-cells of Mougeotia, in the median line of 
which the single mass of chlorophyll appears to float freely, 
surrounded by a colourless homogeneous fluid. This latter, 
however, is found to consist of a mass of closely placed, rather 
elongated, or often somewhat cubical cells, by the expansion of 
which in a spiral. direction the twisted form which the chloro- 
phyll often exhibits is produced. 
These cells likewise usually become very distinct in the joint- 
cells undergoing enlargement alongside wounded joint-cells, after 
they have remained for some time in pure water, and thus give 
the surface of the plant a largely cellular reticulate aspect. 
In Pl. VI. fig. 39, an instance is figured of the expansion of 
one of these endogenous cells to half the dimensions of the 
joint-cell, without having new cells formed within its cavity. 
The great delicacy of the walls of the endogenously stratified 
(nested) system of these cells is an obstacle to the recognition 
of the plan of their arrangement and of the course of their de- 
