298 C aider .—Studies on the Chlor op lasts of Desmids. II. 
In those species of the genus having the thickest ceils the ridges are 
comparatively speaking very large and profusely branched. Their arrange- 
ment is usually very irregular, and they extend in all directions towards 
the cell-wall (Figs. 19 and 25). Here the axial part of the chloroplast is 
quite insignificant, and it is not surprising that in the apical and median 
regions of the cell the axis is often wanting. 
It is probable that this absence of chlorophyll from the apical region 
of so many individuals is a manifestation of the attempt on the part of the 
organism to fill the more peripheral parts of the semi-cell with photosyn- 
thetic material as soon as possible, at the risk of robbing the interior of its 
normal share, since the parts of the chloroplast underlying the cell-wall 
are obviously more important in the process of photosynthesis than the 
more internal parts. This theory is supported by observations of the 
behaviour of the chloroplasts during cell-division. When the young semi- 
cell is newly formed it contains at first no chlorophyll, but soon the 
chloroplast of the older semi-cell begins to bud, sending two lobes through 
the isthmus, one on each side of the nucleus into the young semi-cell. In 
the very flattened cells of some species this bilobed form is not retained 
in a very conspicuous manner as the chloroplast streams more and more 
into the new semi-cell, and very soon the median part between them also 
begins to bud, so that the ingrowing chloroplast becomes practically semi- 
circular in outline, excepting for a small gap often found in its extreme 
apex. This is because all parts of the cell are equally good for photosyn- 
thesis, and it is just as important that the median part of the cell should 
be provided with chlorophyll as the more lateral parts. But in the thicker 
celled species the two small lobes first projected into the new semi-cell 
quickly expend their substance in an attempt to mantle the cell-wall by 
the formation of numerous large ridges which spread out in all directions 
as more and more chloroplast enters from the older semi-cell. Whilst this 
is happening the axis has very little chance of developing in the median 
part of the cell, as indeed there is little need for it to do so, since the 
median part of the front walls is probably already sufficiently mantled by 
ridges extending from the left and right lobes of the chloroplast. Thus it 
often happens that when the division of the chloroplast occurs at the 
isthmus, the chloroplast in the new semi-cell is still distinctly bilobed, or, 
if the growth of the two chloroplast lobes has been so rapid that the axis 
in the median part has not been able to develop at all, then these two 
lobes become completely isolated when separated from the original 
chloroplast by the division at the isthmus, and consequently the new semi- 
cell contains two distinct chloroplasts instead of one. 
The pyrenoids in the chloroplasts of this genus are often extremely 
numerous (Figs. 2, 9, and 17), but at the same time they are very variable 
both in size and number. In individuals of the same species, for instance, 
