222 Carter . — Studies on the Chloroplasts of Desmids . /. 
Thus it is possible for considerable variation to occur in the form of the 
chloroplast within the limits of a single species, according to the relative 
amount of chlorophyll-bearing material present. 
The general appearance of the chloroplast seems to be also greatly 
influenced by the amount of starch contained in it as free stroma-starch 
apart from that occurring in immediate connexion with the pyrenoids. 
This difference can sometimes be distinguished in the living condition, but 
it is much more evident after staining. 
Some individuals stain much more deeply than others, whilst at the 
same time the form of their chloroplasts is much more definite, the various 
plates of which they are composed being very distinct and sharply defined, 
their pyrenoids are conspicuous as dark globules with very definite starch- 
sheaths, and on examining the more minute structure of the chloroplast its 
protoplasmic reticulum is seen to be very fine and close in texture, its whole 
substance appearing homogeneous, except for the pyrenoids. 
On the other hand, certain individuals present a very different appear- 
ance. Their chloroplasts do not stain nearly so deeply and are much more 
bulky, not having the fine definition of the first type, whilst the distinction 
between chromatophores and general cytoplasm is not nearly so marked as 
before. Although there is a paler region round each deeply staining 
pyrenoid, a really definite starch-sheath cannot be distinguished, since there 
is a gradual transition to the general substance of the chromatophore, which, 
when closely examined, is seen to have a very granular appearance, instead 
of being practically homogeneous as before. 
On testing with iodine it is found that the first type of individual con- 
tains practically no starch except that which forms the starch-sheaths of its 
pyrenoids, whilst in the second type the chloroplasts are absolutely packed 
throughout with starch, and it is the presence of innumerable starch-grains 
which causes the granular appearance of the chromatophore described 
above. It seems that under certain conditions starch-grains are thrown off 
from the pyrenoids in large numbers, and spread into every part of the 
chromatophore, stretching apart the meshes of its fine protoplasmic reticulum 
and causing the chloroplast to swell to three or four times its former bulk. 
During the process of becoming swollen with stroma-starch the chloroplast 
often naturally loses its delicate form and becomes a comparatively shape- 
less mass. As a direct result of this, the more delicate parts of its structure, 
e.g. the numerous finger-like projections sometimes found on the external 
surface of many parietal chloroplasts, may be partly or entirely obliterated. 
The distending of the chloroplast begins in the region of the pyrenoids and 
gradually extends to the more remote parts. This phenomenon sometimes 
causes remarkable variations in the form of the chloroplast, -particularly in 
certain species of Closterium , in which the relative size of the central axis 
and radiating ridges is largely dependent on the amount of starch present. 
