386 Ee eA Ne 
one of the most important and prolific groups is that of the diatoms. 
These plants possess a delicate and often very elaborately constructed 
skeleton of silica, and contain a brown colouring matter, which like the 
green chlorophyll of land vegetation is able in the presence of sunlight 
to assimilate the carbon from carbonic acid gas. 
It has formed an interesting subject of research to endeavour to 
ascertain as exactly as possible the conditions necessary for the growth 
of these diatoms, and to find out by means of experiments in the labora- 
tory how the amount of growth may be increased. In order to carry out 
such experiments with exactitude and to obtain precise and definite 
results it is necessary to work with cultures of a diatom which are as 
nearly as possible pure, that is to say, cultures which contain no other 
living organisms excepting the single species of diatom upon which the 
experiments are being made. If two or more living organisms are pre- 
sent in the experimental cultures the results at once become complicated, 
since a second organism not only uses up the different constituents in 
the culture solution, but it also excretes waste products of its own, which 
become dissolved in the water and may affect either favourably 
or unfavourably the growth of the diatom which is being experimented 
upon. 
The experiments had, therefore, to be conducted as nearly as it was 
possible under sterile conditions. The glass flasks in which the cultures 
were made, after being carefully cleaned, were baked in an oven, and 
all the culture solutions were boiled before being used. 
The particular diatom upon which the experiments were made is one 
that is found in the Plankton, that is to say amongst the large number of 
microscopic organisms which float freely in the waters of the sea, and are 
drifted about at the mercy of wind and current. Such organisms are 
collected by dragging through the water a bag-shaped net, made of muslin 
or fine-meshed silk, which strains them out. This form of net is generally 
known as a tow-net. The organisms collect in a tin or bottle attached 
to the end of the net, and can be brought alive to the laboratory in a 
bottle of sea-water. 
The species of diatom used in the experiments is called Thalassiosira 
gravida, Kach diatom cell consists of two flat, cup-shaped valves, fitting 
one into the other and enclosing the protoplasmic substance of the cell. 
The whole cell of Thalassiosira looks something like a flat, shallow pill- 
box. A number of these cells are joined together by threads, which run 
from the middle of one cell to the middle of the next, so that long chains 
are formed. These chains of diatoms have much the appearance of a 
number of buttons strung on a thin wire, with a considerable interval 
between successive buttons. This chain formation is of some importance 
