296 Carter . — Studies on the Chloroplasts of Desmids. II. 
species, according to the physiological condition of the cell. This is 
particularly the case with some of the flat-celled species, where frequently 
in some individuals no trace of ridges is to be found at all, whereas in 
others they are quite distinct. In other thicker celled species, e. g. 
M. truncata , they are often so large, numerous, and profusely branched, 
that the axile plate is frequently quite hidden by them. The size and 
prominence of the ridges depends largely on the thickness of the cell, 
because they stretch out towards the cell-wall, chiefly at right angles to the 
axile plate. 
In the flat-celled species of Micrasterias the axile plate does not 
usually occupy more than one-third the entire space between the two front 
walls of the semi-cell (Figs. 3-8 and 10), but the thickness of the chloroplast 
always varies considerably according to the amount of stroma starch 
contained in it. The axile plate extends into all the peripheral lobes and 
projections of the cell-wall, and is slightly hollowed out at the base of the 
semi-cell for the accommodation of the nucleus (Figs. 2 , 9, and 24). In 
outline it may be very similar to the cell-wall, or it may be even more 
intricately incised and lobed. Sometimes it seems to be drawn out at 
intervals into threadlike strands vvhich apparently connect up the chloro- 
plast to the peripheral layer of cytoplasm lining the cell-walk This 
happens when the chloroplast is not sufficiently massive to extend right up 
to the cell-wall at all points. 
In those species with very thick cells the axile plate is not so evident 
in the front view as in the case of those species having flattened cells 
(Figs. 13, 19, and 25). It is seen, however, in the end view or in transverse 
sections as an extremely thin plate lying in the middle of the cell, half- 
way between the front walls (Fig. 27). Towards the sides of the cell it 
usually loses its identity in the profusion of branching ridges (Figs. 14, 20, 
21, and 27). Very often in such cases it has no definite plate-like form, but 
consists of a more or less irregularly shaped mass connecting up the huge 
ridges one with another, these forming by far the more important part of 
the chloroplast (Fig. 21). 
In nearly all the species examined a more or less extensive hollowing 
away of the central axile plate was noticed in the upper region of the polar 
lobe. This is often seen as a semicircular or more elongated space free 
from chloroplast in the extreme apex of the cell. In the flat-celled species 
this apical hollowing rarely extends for more than one-fourth the distance 
from apex to nucleus, and is frequently less (Figs. 2 and 24). In the thick- 
celled species, however, the axile plate is sometimes shortened from the 
apex by as much as two-thirds its greatest length, so that the chloroplast 
is distinctly bilobed (Figs. 15, 19, and 25). When this happens the ridges 
often spread round over the internal surface of the wall so that the absence 
of the axis in the apical region may not at first sight be noticed, but in 
