f 
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210 AMERICAN MIDLAND NATURALIST. 
mals. At _— such large several pieced vessels seldom ex- 
plode, or crack. 
Microscopic plants ae iere to subtle influences of 
environment which ar ostly unknown to us. The plant 
aquarium must be ania of glass or other absolutely Argent 
ble material like porcelain. Metals in contact with water in 
plant jars are fatal. Carbon dioxide present in small traces 
in water where living things are growing will dissolve minute 
oe e otherwise insoluble metallic compounds, such as 
ust or oxides, and carbonates resulting in the formation of 
si or leds soluble Bicarbonates. Soluble metallic salts are 
ionized in water and therefore toxic. These substances may be 
almost imperceptible to analysis or at least but traces, but 
there is enough to prove fatal to plants. Even tap water may 
contain too much iron in solution. Many microscopic plants 
seem to be killed off by iron almost as easily as by lead, and 
this would seem all the more strange as iron is a necessary 
constituent of chlorophyl. Other plants appear to resist the 
action of iron only because they are able in some way to pre- 
cipitate it from solution as a sulphide, and thus render it, 
This can be easily shown in jars when Protococ- 
coideae abound. The walls of the vessel become coated with a 
brownish layer which can easily be washed off with dilute 
acids when hydrogen sulphide comes off. The conjugatae, es- 
pecially Spirogyra, do not seem to be able to precipitate iron 
nor resist its toxic action. Not being able to get rid of it they 
promptly die s 
It is not true that larger aquaria are best for growing al- 
gae, though such are unquestionably most desirable for ani- 
mal development. My experience of some years past is that 
the valuable plants for class demonstration are grown in glass 
vessels of medium size of less than a gallon capacity and not 
exceeding two gallons. The very best plants, and often in 
great variety, have been growing in vessels holding little more 
than an ordinary tumbler carefully covered e prevent evapor- 
ation and access of dust. I have at present in the laboratory 
the best variety of desmids I have ever seen in a laboratory 
culture. The vessel is shallow and wide and when full, holds 
not quite a pint of water. It has stood covered for months 
and contains innumerable specimens of Staurastrum, as also 
several species of Closterium and Cosmarium. , These are con- 
stantly dividing. 
Another jar has stood in the laboratory for about six 
months, having been filled with some Spirogyra and other con- 
jugatae from the field, together with some sticks and debris 
