292 PRINCIPLES OF SOIL MICROBIOLOGY 



a sterile moist chamber. After growth has taken place, the slides are dried, fixed 

 in alcohol and stained. The entire colony, with both vegetative and aerial 

 mycelium, can thus be examined in an undisturbed condition. 



2. Method of Drechsler: The organism is grown on a synthetic medium and, 

 when the culture is fully developed, the whole colony is cut from the agar and 

 removed, as carefully as possible, from the tube or plate. A slide smeared with 

 albumin fixative is now brought into firm contact with the surface mycelium 

 and then separated from it, precautions being taken to avoid any sliding of the 

 two surfaces on each other. If the growth is not too young, the upper portions 

 of the aerial mycelium will be left adhering to the slide without any series dis- 

 arrangement; killing and fixation may be effected at once. The material is 

 then stained and mounted in balsam. Preparations, in which the spore chains 

 have commenced to disintegrate, are impaired by the large masses of free spores. 

 The most convenient fixative agent is 95 per cent alcohol. As a stain, Maiden- 

 hain's iron-alum haemotoxylin is good for protoplasmic structures. Delafiold s 

 haemotoxylin, allowed to act for 24 hours with the proper degree of decoloriza- 

 tion, yields deeply stained, clear preparations showing distinctly the vacuoles, 

 metachromatic and nuclear structures, and septa. 



Nature of growth on artificial media. The term colony is used incor- 

 rectly in designating a mass of growth of an actinomyces, since it is 

 merely a mass of mycelium developing out of a single spore, and not a 

 colony in the sense of bacterial growth. Each spore or piece of mycelium 

 separated from the colony is capable of individual existence, develop- 

 ing into a new colony. The single-celled colony of an actinomyces is 

 characteristic and is easily distinguished from that of bacteria or fungi. 

 It is usually round and develops in the form of a semi-circle into the 

 medium (No. 120, PI. XVII). The colonies are mostly compact, leathery, 

 adhering to the medium, the surface being either flat or elevated; the 

 outer zone is smooth, round as seen with the naked eye, and has a fringe 

 of minute hyphae projecting for a short distance into the medium when 

 observed under the low power. The surface is usually dry and often 

 presents a conical appearance; it is either free from any aerial mycelium, 

 or covered with a chalky (mealy, mildewy) white, drab or grey aerial 

 mycelium, or with an abundant cottony, fuzzy, white, red or grey aerial 

 mycelium. 



The subsurface growth of different organisms is little differentiated 

 from one another, being usually of white-greyish or yellowish color ; but 

 the surface growth and the subsurface growth which may develop up to 

 the surface, have a characteristic appearance on synthetic media. The 

 growth of some organisms presents a smooth surface, while others have 

 a much folded, or lichnoid surface; still others form a fine network on the 

 surface. These characters are not constant but change with the composi- 

 tion of the medium and age of culture even on artificial culture media. 



