THE STRUCTURE OF PROTOPLASM 270 
solved and transported throughout the cell and the tissue or organ and to 
more or less distant parts of the plant. That the plastid is to be regarded 
as a region of the protoplasmic complex rather than as a differentiated and 
definitely delimited body is shown with especial clearness in the case of 
those algae whose chloroplasts are of irregularly lobed or frayed-out outlines. 
The functions of the chloroplast in forming assimilation starch are strictly 
dependent on the presence in it of green chlorophyl, and cytologically the 
chloroplast is perhaps little more than an area of the cytoplasm impreg- 
nated or infiltrated with chlorophyl. The leucoplast is obviously less 
differentiated than the chloroplast. That in the marginal lobes of the 
chloroplast the green pigmentation seems frequently to pass over by an 
imperceptible shading off in color into the gray of the adjacent cytoplasm 
is an observation easy to make on many of these algae. Hydrodictyon is 
especially well suited to illustrate the relations of the starch-bearing region 
to the remainder of the protoplasmic mass. The earlier writers had at- 
tempted to recognize a zonal differentiation in the Hydrodictyon cell, and 
the inner green zone or plastid with its pyrenoids was described as being 
free from nuclei; but Timberlake's sections show that pyrenoids and nuclei 
are scattered through the whole thickness of the primordial utricle about 
the central vacuole. Presumptively at least the whole primordial utricle 
is infiltrated with chlorophyl in the mature cell. The appearance of the 
cells of the young and growing nets of Hydrodictyon is also very suggestive 
as to the nature of plastids and their delimitation. As my photographs 
show, and as has been noted before, the ends of the young cells are gray 
and free of green chlorophyl, the latter occupying a band-shaped and very 
vaguely delimited zone in the middle region of the cell. As tiie nets grow 
larger, however, the chlorophyl spreads toward the ends of the cells until 
they are uniformly green in color, the number of pyrenoids increases from 
one to several, and the appearances suggest not the growth of a specific 
body but the increase of materials concerned with starch formation, pyre- 
noids and chlorophyl, and their gradual spread throughout the whole 
primordial utricle. This spread and increase of the pyrenoids involves 
their division, but the chlorophyl apparently merely diffuses out into the 
adjacent colorless regions of the cytoplasm as it increases in amount. In 
Botrydium, for example, and in the higher plants the chloroplasts appear 
as quite sharply delimited bodies of definite form and outline which them- 
selves, at least at certain stages, arise by division, though their transmission 
as such through the egg and pollen tube may still be regarded as in question. 
It has not been adequately shown in any case, however, that the plastid has 
a specific membrane of its own like the plasma membrane or that of the 
nucleus. It is, so far as the microscope shows, a green-pigmented, denser 
region of the cytoplasm — a group of elements of the polyphase colloidal 
cytoplasmic system. The absence of a specific membrane about the 
plastid is shown especially by the widely recognized occurrence of stroma 
starch. 
