246 Carter . — Studies on the Chi or op lasts of Desmids . /. 
X. acideatum , X. acanthophorum , and X. hastiferum. 
These three species were found to have axile chloroplasts. In all cases 
there are two chloroplasts in each half-cell (Figs. 117, 118, and 127). Each 
chloroplast is provided with an axis which arises on one side of the isthmus 
near the nucleus and extends obliquely towards the corresponding lateral 
region of the semi-cell, increasing in size and containing typically one large 
pyrenoid ; cf. Fig. 127 (lower semi-cell). 
Around this axis are arranged a varying number of plates which radiate 
towards the cell-wall in various directions. Four of these plates are usually 
more prominent than the others, one extending to each front face of the 
semi-cell, and two to the corresponding lateral face (Fig. 128). Other 
plates or ridges, if present, are often much more irregular, and do not 
extend from end to end of the semi-cell in the same way as the four larger 
ones (Figs. 317 and 118). The margins of the plates may be quite smooth, 
irregular, or lobed. 
X Brebissonii. 
In this species the chloroplasts in typical specimens are exactly 
similar to those of X. acideatum , X. hastiferum , and X. acanthophorum , 
there being in each semi-cell two axile chloroplasts each with a single 
pyrenoid (Figs. 119, 120, and 121). In many cases, however, the pyrenoids 
are much more numerous, as many as nine being present in one semi-cell, 
and the form of the chloroplasts may be considerably changed. Very often 
there are two or three pyrenoids in the axis of each chloroplast where 
typically there should be but one (Figs. 122 and 123), or again there may 
be two or three very small pyrenoids in the peripheral edges of one or more 
of the plates radiating from the axis (Fig. 123). The actual form of the 
chloroplast in such cases is often not seriously interfered with, and there 
are as before two axile chloroplasts in each semi-cell. In other individuals 
the numerous pyrenoids are not confined to the centre of the chloroplast, 
but, on the other hand, pyrenoids of considerable size may also occur in the 
more peripheral parts of the cell, showing the increased importance of the 
parietal parts of the chloroplast even although as a whole it may still be 
axile (Fig. 124). In other specimens the pyrenoids are quite absent from 
the positions in the interior of the cell in which they should typically occur, 
and the form of the chloroplast is quite different, being practically parietal, 
often very irregular in shape, consisting of a variable number of masses, each 
of which contains several scattered pyrenoids (Fig. 125). Occasionally there 
are four definite parietal plates each with a pyrenoid, regularly arranged, as 
in those species of the genus which typically have parietal chloroplasts 
(Fig. 126). 
The axile condition of the chloroplast seems to be far more common 
than the parietal, and of sixteen specimens examined only five exhibited 
