310 Carter . — Studies on the Chloi'oplasts of Desmids. IV ’ 
St. tumidum. 
This species differs from all the other species of the genus examined in 
having chloroplasts which are probably invariable in their parietal disposi- 
tion. There are usually about twelve to fifteen or sometimes more chloro- 
plasts in each semi-cell, and they are in the form of rather narrow bands, 
running longitudinally, each with two or three pyrenoids embedded in it 
(Fig. 48). Very often the bands are fairly even in width throughout their 
length, their outlines are often slightly irregular, but their structure does 
not seem to be complicated by the presence of projections from the edges 
and surface, as in the case of the parietal chloroplasts of some other genera. 
In other specimens the chloroplasts are drawn out at certain points through- 
out their length to form very thin strings between the large globular masses 
which alternate with them and contain the pyrenoids, whilst in other cells 
the bands are irregular in form and do not extend from end to end of the 
semi-cell (Fig. 47). 
XII. The Behaviour of the Chloroplasts during Cell-division. 
■ ■■ • ■ 
The general external appearance of cell -division in Desmids was first 
figured by Ehrenberg (1838) in Cosmarimn^ and. later in other genera as 
well by Focke (1847), Ralfs (1848), Nageli (1849), de Bary (1858), and 
Delponte (1873). In most cases, however, these investigators did not 
attempt to illustrate the origin of the chloroplast which eventually becomes 
apparent in the new semi-cell, and it is only in the figures of Focke (1847) 
for Micrasterias , and de Bary (1858) for Cosmarium Botrytis , that any clue 
is given to the behaviour of the chloroplast during cell-division. Amongst 
unconstricted Desmids the details of cell-division, including the division of 
the chloroplast, have been thoroughly investigated in Closterium by Fischer 
(1883) and Lutman (19115, and thus, although our knowledge of the 
processes accompanying cell-division in the Saccodermae is fairly complete, 
in the much larger group of constricted Desmids nothing has been discovered 
since the time of de Bary, and there is no definite information concerning 
the division of the chloroplasts in these forms, with the exception of the very 
slightly constricted genus Hyalotheca , in which cell-division has been 
recently studied by Acton (1916). 
In Hyalotheca and Closterium^ the only two genera which have at 
present been investigated, the process of cell-division is not identical. In 
Closterium , as described by Lutman, the first intimation that cell-division is 
about to take place is seen in the chloroplasts, which show a pinching-in 
about one-third the distance from nucleus to apex. Several hours later the 
nucleus divides, and eventually a transverse wall is laid down between the 
daughter nuclei. The latter then become amoeboid and begin to move 
away in opposite directions, travelling towards the constrictions in the 
