Development, and Structure of the Vegetable Cell. 279 
It would appear that those materials which are subservient to the 
growth of the young cell-wall pass from the lower to the upper 
portions of the plant, whilst chlorophyll is produced from com- 
pounds which diffuse themselves from the upper into the lower 
parts of the plant. 
During the first period, which ensues upon the unfolding of 
the annular fold and the extension of the cells, the upper young 
joint-cell usually remains more or less unchanged; the lower 
one, on the contrary, continues to enlarge, whereby the new and 
still delicate transverse septum becomes pressed upwards, until 
it at length projects from the opening in the lacerated mem- 
branous sheath, and then it proceeds to increase in thickness. 
(Pl. V. figs. 23, 24.) 
Bary asserts that he distinctly made out that the septum is pro- 
duced by a gradual constriction and secretion of the primordial 
layer, in various species of CEdogonium, after the protrusion of 
the clear lamina from the membranous sheath. In the treatise 
quoted (at p. 42) he says:—“In any case, the septum is not 
simultaneously formed in its entire surface; on the addition of 
a solution of chloride of zinc and iodine, the contracted primor- 
dial layer is sometimes distinctly seen to pass through the 
middle of an incompletely closed dissepiment.” 
That this phenomenon, which I have figured in a Spirogyra 
(Pl. VII. fig. 67), and to which I shall hereafter revert in con- 
nexion with Cladophora, furnishes no sufficient proof of the pro- 
duction of 'the dissepiment by constriction, will be shown when 
I come to speak of the latter genus. 
Of the formation of a fold in the membrane of the mother 
cell, after the two daughter cells (figs. 21, 26, 28) have become 
mutually pressed together so as to form a complete septum 
(figs. 22, 27, 29), and of an eventual inward extension of this fold 
betwixt the lamelle of the septum, which are thus again sepa- 
rated, I have no knowledge; but the more distinct appearance 
of the long-previously existing septum can, in my opinion, be 
much more readily accounted for by the gradual thickening that 
progresses in it from its circumference. That this thickening 
of the septum always follows after its emergence from the enve- 
loping sheath is in all probability to be attributed to the changes 
in the nature and operation of the nutritive matters derived from 
without, now only through the walls of one assimilating and 
secreting system of cells. 
In the mean while, the upper of the twin cells also begins to 
grow vigorously, until at last it equals the lower one in length. 
Nevertheless it not unfrequently remains somewhat shorter, 
which is the cause of the irregularity seen in the structure of 
Cdogonium, 
