52 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
watch-case milling, hexagonal network, the interspaces of which are 
often further ornamented with secondary sculpture, intricate ara- 
besque designs, in short a diversity and delicacy of embellishment 
that makes these plants the most ornate of all living objects. The 
highest efficiency of the microscope is often taxed in revealing some 
of the minuter markings; and valves that were formerly thought to 
be quite smooth, and which, therefore, bear the inappropriate name, 
“hyaline” or “pellucida,” are now under better objectives found to 
be intricately carved with intersecting lines. It should be here 
stated that the few illustrations accompanying this description con- 
vey no adequate idea of the objects they represent; for in black 
and white it is quite impossible to reproduce the appearance of 
these structures of shining silica, often further beautified by pris- 
matic colors refracted from their various surfaces. 
Outside of this silica casing there is a very thin and perfectly 
transparent organic pellicle, in vital connection with the living sub- 
stance within, erroneously called a gelatinous sheath. It is this 
internal living substance which determines the position of the organ- 
ism as a plant, and which presents some of the most interesting prob- 
lems connected with its life-history. It is made up of normal plant 
protoplasm (cytoplasm) with a single large centrally placed nucleus. 
Rarely there are two nuclei, found only in two or three species, where 
they are said to be constant. Generally there are two large vacuoles 
filled with cell-sap, and two or more chloroplasts. These latter are 
usually symmetrically arranged, in the elongated diatoms on either 
side of the median line and in the circular forms in evenly distributed 
granules or larger masses radially disposed. The green chlorophyl 
composing these bodies is disguised by an overlying brown or buff 
pigment called diatomin, which is so readily soluble in alcohol that 
when living diatoms are treated with that liquid the diatomin in- 
stantly disappears and the plants are seen to be bright green. The 
reserve food material stored up by the diatoms is not in the form of 
starch grains, but of globules of deep yellow, dense and highly re- 
fractive oil, either floating in the sap of the vacuoles or embedded in 
the cytoplasm. 
Turning now to the physiology of the diatoms the question of their 
nourishment may be considered. This takes place as in other chloro- 
phyl-bearing plants, by the assimilation of inorganic substances in 
solution in the water about them through the agency of sunlight in 
conjunction with the chlorophyl masses; and in consequence of this 
fact it is plain that they are precluded from such waters as are not 
sufficiently lighted; as, for example, subterranean streams and the 
