CULTIVATION AND EXAMINATION 41 



and a loopful removed to a second tube, followed by the same manipulation 

 a third or fourth time. The higher dilutions are then poured into petri 

 dishes where colonies develop. This method has all the faults of the water 

 blank dilution technique. The procedure has been widely used but has not 

 been followed in our study of the Aspergilli. Blakeslee (1915) and other 

 mycologists have prepared such dilutions in bottles by slowly rotating the 

 bottle in a cold water or ice bath, thus allowing the agar to congeal in a thin 

 layer against the glass surface. This practice has little to recommend it: 

 (1) the resulting colonies could be isolated more readily from a petri dish, 

 and (2) it is basically impractical to make satisfactory observations regard- 

 ing the character and structure of a mold colony, or the gross features of its 

 fruiting structures, when observations have to be made through the curved 

 and non-uniform walls of a glass bottle. 



Smears and Streak Cultures 



The practice of smearing a suspension of mold spores over the whole plate, 

 or the whole length of a slanted agar tube, results in a growth of mycelium 

 which covers the entire surface and usually produces a greater mass of co- 

 nidia. But individual colonies are usually unidentifiable in such preparations, 

 hence they are of little value in providing those critical details of colony 

 habit, coloration, and texture necessary for identification and classification. 

 In general, mycelium derived from different conidia of the same strain will 

 intertwine without inhibition, and even anastomose, if they come into con- 

 tact in the early stages of growth; for example, before any sign of conidium 

 production appears. If the spacing of the spores is great enough to permit 

 the establishment of small fruiting colonies before such contact is made, the 

 colonies frequently do not converge and complete coverage of the surface of 

 the agar with the production of maximal quantities of conidia does not oc- 

 cur. Closely-placed seeding of spores is useful to obtain the largest possible 

 supply of conidia; but to study the normal characters of a species, individual 

 colonies must be allowed to develop without interference by others of the 

 same or different species. 



Streak cultures can be employed to advantage in freeing one mold from 

 another, or from other contaminating organisms. By touching a sterile, 

 moistened needle to a single conidial head or other selected fruiting surface 

 of limited extent, and subsequently streaking this repeatedly across the 

 surface of an agar plate, isolated colonies of the desired species can usually 

 be obtained. Occasionally it is desirable to streak two plates in succession 

 in order to secure a satisfactory separation of colonies. In any case the 

 spore source should be selected with great care and the use of a low-power 

 binocular microscope or pocket magnifier is recommended. When this 

 method is employed, it is desirable to reisolate from these individual colo- 



