302 Carter. — Studies on the Chloroplasts of Desmids. II. 
ridges stretching towards the cell-wall in all directions (Fig. 21). In the 
front view two median ridges are usually quite conspicuous in comparison 
with the others, and that part of the central axis lying between them, even 
where it is present, is often very delicate and only found after careful 
focusing (Fig. 25). Occasionally specimens of M. truncata have been 
observed in which two distinct chloroplasts were present in each semi-cell, 
the axis in the median region of the cell being altogether absent, but as 
a rule the remains of the axile plate can be seen just above the nucleus. 
When there is a very extensive hollowing away of the axis in the 
upper part of the semi-cell, the ridges in the apical region often spread 
themselves closely over the inner surface of the cell-wall, forming a more or 
less definite parietal layer (Figs. 25 and 26). 
The pyrenoids in both species vary very much in size, and usually 
number about five to ten in each semi-cell, although there may be as many 
as twenty. They are usually to be found embedded in the thicker parts 
of the chloroplast, where the ridges arise from the central axis, or even in 
the more peripheral parts of the ridges themselves. Occasionally pyrenoids 
occur in the parietal part of the chloroplast in the apical lobe where the 
axis in this region is wanting (Fig. 26), but the majority of the pyrenoids 
are always to be found in the interior of the cell. They are very often seen 
to be in a state of active division, two or more small ones being constricted 
off from a single large one. 
Summary of the Special Characters of Micrasterias . 
In all the species examined there is normally one chloroplast in each 
semi-cell, and all the different forms encountered can be considered as 
variations of one simple type. 
There is always a more or less distinct axile plate parallel to the front 
faces of the semi-cell in the central position, and from this there usually 
arise numbers of ridges or plates which project towards the cell-wall in 
various directions. The relative size of the axis and ridges varies with 
different species. 
The prominence of the ridges seems to depend on the thickness of the 
cell ; in the more flattened cells of some species they are insignificant and 
may under certain physiological conditions be altogether absent, but in the 
thicker celled species they are very large and may sometimes even be 
branched. The ridges in the latter case constitute by far the more 
important part of the chloroplast, the axis often becoming very thin and 
indefinite in form. 
With the increase in size and importance of the ridges, the pyrenoids 
become more restricted in number and disposition, occurring only in the 
more massive parts of the chloroplast. 
