1 6 MORPHOLOGY OF THE CELL. 
B. Cell-formation in Growing Organs: Vegetative Cell-formation. 
This mode of cell-formation consists almost invariably in the bipartition of a mother- 
cell ; there is no or scarcely any perceptible rounding off or contraction of the dividing 
protoplasm ; the two daughter-cells entirely occupy the place of the mother-cell. 
I. The partition-avail is formed gradually. The protoplasm of the mother-cell projects 
in the plane of division as a circular protuberance, and a ridge of cellulose is formed 
in it which finally developes into a complete septum. 
A clear and well-studied example 
is afforded in the stouter species of 
the ^^mx^ Spirogyra^. In order to 
observe the divisions, it is necessary 
to place filaments that are in active 
growth in very dilute alcohol after 
midnight, that they may be exam- 
ined by day, the divisions taking 
place only by night. Fig. 13 A 
shows a living cell of a filament of 
S. longata by day \ B \.o E the stages 
of division at night ; the protoplasm 
of the cells is contracted by the 
alcohol. 
[The phenomena of division fall 
into two groups, namely those which 
attend the division of the nucleus, 
and those which are connected with 
the ingrowth of the partition-wall. 
The nucleus of a cell which is about 
to divide becomes broader, assum- 
ing the form of a biconcave lens, 
and its nucleolus breaks up into 
irregular granules, which, together 
with its other granular contents, 
begin to form a nuclear disc in the 
equatorial plane. A delicate stria- 
tion is now apparent in what is 
becoming the long axis of the nu- 
cleus, at right angles to the nuclear 
disc, and a characteristic nuclear spindle is gradually produced. The nuclear disc splits 
into two halves lying side by side, each of which travels to the corresponding pole of 
the nucleus ; thus two new nuclei are constituted which are connected by fibrillae. 
The first indication of the formation of the partition-wall is the accumulation of 
protoplasm at about the middle of the length of the cell, so as to form a slight annular 
protuberance into the sap-cavity: this takes place about the same time as the first 
changes in the nucleus. Within this protuberance a rim of cellulose is formed which 
grows inwards, carrying before it the spiral chlorophyll-band (Fig. 13, JB and C) ; 
soon the chlorophyll-band is cut through, and margins of the ring of protoplasm 
coalesce in the middle so as to form a plate, to which the fibrillae connecting the two 
nuclei, which have been formed in the meantime, become attached; in this plate the 
remainder of the cellulose wall is formed. The nucleus of each cell now travels to 
Fig. T.j,.—Spirflgyra longata (X 550). A a cell in the living state; 
B, C cells laid in dilute alcohol during the division by night; D, E central 
portion of cells in the act of division. 
This case was the first of all the processes of cell-formation that was accurately examined : 
H. von Mohl first described it in 1835 Conferva glomerata. Mohl, Vermischte Schriften bot. 
Inhalts, Tübingen 1845 : [Strasburger, Zellbildung und Zelltheilung, 1880.] 
