248 Carter . — Studies on the Chloroplasts of Desmids. /. 
In practically all cases each parietal chloroplast is in close connexion 
with the nucleus at the base of the semi-cell ; cf. Fig. 106. 
With the exception of the large species X. armatum , there is typically 
one pyrenoid in the centre of each parietal chloroplast (Figs. 106, 109, 113, 
and t 1 5), although very often two or three may be seen close together in 
one or more of the plates (Fig. 108). The pyrenoids are often very large, 
and the thin chloroplasts are in consequence thickened considerably where 
they occur, the corresponding chloroplasts of the two front faces of the 
semi-cell sometimes projecting so far into the interior of the cell that they 
nearly touch at these points; cf. Figs, no and 111. In X. armatum the 
pyrenoids are more numerous, numbering about five to twelve in each 
chloroplast (Figs. 129-32). They vary considerably in size, some of them 
being extremely small, and comparable to the small globules found in the 
parietal parts of the chloroplast in Cosmarium ochthodes and other species. 
VIII. Summary of the General Characters of 
the Chloroplasts. 
The chloroplasts in the Saccodermae are, with a few exceptions, usually 
very simple in form, and are nearly all well known. 
In the higher Desmids the chloroplasts are, in general, very compli- 
cated and beautiful structures, and many of them have not hitherto been 
investigated. The delicacy of their form is dependent upon — 
(a) the relative amount of chlorophyll-bearing substance present in the 
individual, and 
(b) the general physiological condition of the cell, particularly as regards 
the quantity of free stroma-starch present in the chromatophore. 
The chloroplasts may be axile or parietal, and their general form is 
usually constant, but in a few cases there is marked variation in the disposi- 
tion of the chloroplasts amongst individuals of one species. 
The number and position of the pyrenoids present depend on the 
size and shape of the chromatophore. In the more extensive chloroplast 
plates of both axile and parietal forms they are often very numerous and 
scattered. 
In all cases the amount of foo.d stored in the form of pyrenoids is 
dependent on the condition of the cell, and at any time two or more 
pyrenoids may be formed by the division of an original one. 
In many forms the number of pyrenoids present in each semi-cell is 
generally supposed to be constant. This is not true, since the actual 
number of pyrenoids present is dependent on various changing factors, but 
in such cases the points of pyrenoid formation are usually quite definite and 
fixed. 
In addition to the ordinary large pyrenoids which are usually provided 
