22 E. G. PRINGSHEIM 



that walls of glass receptacles remain clean. The destruction of 

 contaminating organisms is performed by heating in a steam 

 chamber for an hour on two consecutive days. When material 

 such as peat or wheat which are not easily soaked are used, pre- 

 vious heating in a water bath or the employment of boiling water 

 prevents particles from floating up and soiling cotton plugs. 



Isolation 



The soil-water technique is meant to be used for uni-algal cul- 

 tures. If mixed populations from nature are introduced, those 

 species which grow rapidly soon prevail, whereas the more exact- 

 ing, and often interesting ones, are found at best only at the 

 beginning of the culture period. In order to free the more selec- 

 tive species from competition, a mechanical isolation with capil- 

 lary pipettes under the microscope has been worked out. A binoc- 

 ular dissecting microscope can be used, but is not always suffi- 

 cient to distinguish the species owing to its poor optical perform- 

 ance and low power of magnification. An instrument with con- 

 denser and ordinary low-magnification objectives equipped with 

 porro prisms to erect the image has been constructed. This is far 

 superior in color differentiation and resolution, so that, for in- 

 stance, species of Eugleiia, Trachelomonas, and Chlamydomonas 

 can be distinguished when sv/imming in a watch glass. Its lower 

 working distance, however, calls for more practice in handling 

 the tip of the capillary than does the ordinary binocular. 



Pipettes are made of soft glass tubing with a 3-4 mm. bore. 

 Hard glass has no advantage and is more difficult to handle. Even 

 organisms from acid habitats are not damaged by alkali from 

 the glass during the short period of manipulation. Lengths of 

 glass tubing of 35-40 cm. are drawn out to provide two pipettes 

 which are plugged with cotton and sterilized singly in larger 

 tubes. Before use, the narrow part is drawn out again over a little 

 flame to about 0.5 mm. bore and at a distance of 6-10 cm. again 

 to a real capillary of about o.i mm. which is cut short by a sud- 

 den pull with a pointed forceps, thus producing a circular, 

 smooth opening. Recent experience has taught that the narrowest 

 part of the pipette should be short in order to give little resistance 



