

320 BOTANICAL GAZETTE [October 



Fig. i 



over the spore in the place of the objective, and the marker is lowered 

 so as to cut out a disk of agar inclosing the spore. The spore may 

 then be examined again with the objective to see that it is really included 



in the agar disk, after which the disk is lifted from 

 the Petri dish with a flattened platinum wire and 

 placed in a culture tube. This method has the 

 advantage of allowing a rapid and accurate location 

 of spores and of guaranteeing that only a single 

 spore is transferred. The disadvantages of expos- 

 ing the culture to the air for rather long intervals 

 is not a great one, because contaminations need not 

 occur if proper precautions are taken. For such 

 work the ringed tip of the marker may have a 

 diameter of about 5 mm. 



A modification of this method has been found 

 of great use in the selection of spores of Pestalozzia. 

 In this case it was necessary, not only to locate single spores accur- 

 ately, but also to measure the spores w T hen located, so that the longest 

 and the shortest might be taken. Finally it was found that this could be 

 done by making dilution cultures, and then spreading the agar of the tube 

 containing the desired dilution in a thin film on sterile glass slides. These 

 slides could then be examined under the microscope, the spores measured, 

 and those of the size desired cut out by the marking device, and finally 

 the disks containing the spores could be transferred to culture tubes. 



The agar for making the dilution cultures needs to be very carefully 

 filtered so as to be as transparent as possible. For the work with 

 Pestalozzia a 1 per cent solution of Liebig's beef extract with 3 per cent 

 agar was used and found satisfactory. The nutrient solution, of course, 

 must be of such a nature as to be suited to the fungus in question. 

 The agar film on the slides is formed by pouring two or three drops of 

 agar on the slide and then spreading them over the entire surface with 

 a sterile needle. Too thin a film cannot be lifted from the slide; too 

 thick a film will not allow the high power objective to be used, which 

 is .necessary with measurements of very small spores, so that some 

 practice is needed to secure good results. I am indebted to Miss 

 Bachmann 3 for the idea of using agar films on slides, although films 

 formed according to her method are too thin for this purpose. In this 

 selection work a marker with a tip 1.5 mm. in diameter was found 

 most useful. — Carl D. LaRue, Kisaran, Asahan, Sumatra. 



3 Bachmann, Freda M., Amer. Jour. Bot. 5:32-35. 1918. 



