146 PLANT PARASITES. 



groups streptothrix, cladothrix, crenothrix, leptothrix may be named. 

 Among these the streptothrix is of most significance here, since patho- 

 genic forms are known. The Streptothrix actinomyces, and closely related 

 species, and the lesions which they induce, w r ill be described later. 

 There are many reasons for the belief that the forms called streptothrix 

 and actinomyces are more closely related to the moulds than to the bac- 

 teria, but the scope of this work does not permit the discussion of the 

 subject, particularly difficult as it is on account of the confusion of 

 terms and the lack of sufficient knowledge of the life history of the 

 organisms involved (see Actinomyces, p. 223). 



Variations in Forms. Their apparently simple structure and the lowly 

 position which bacteria occupy in the scale of living things have given 

 rise to the conjecture that marked changes in form within the limits of 

 the primary groups, or even changes from one primary group to another, 

 may be brought about by alterations in environment, food, etc. In the 

 early days of the exact study of bacteria this belief in pleomorphism in 

 bacteria found ready currency. But the more exact study of separate 

 forms, w r hich the new technique has made possible, has led to the general 

 acceptance of the view that variations do not occur except within com- 

 paratively narrow limits, and that what we are accustomed to call species 

 of bacteria maintain their morphological characteristics with tenacity 

 under the most varied changes in environment, even though these persist 

 through the countless generations which may pass within the limits of a 

 single experiment. The physiological characters of bacteria are, as we 

 shall presently see, subject to wide and significant variation, but, so far 

 as we can now see, monomorphism widely if not exclusively prevails. 



Spores. Under a variety of conditions, the limitation of which are 

 not very well understood, new bacteria are produced, and the species is 

 perpetuated, not by simple division, but by the de- 

 velopment of spores. The most common mode of 

 spore formation is called endogenous. A small, shin- 

 ing mass makes its appearance within the proto- 

 plasm from which it is formed, grows more and more 

 distinct, and finally appears as a sharply defined 

 spheroidal or oval, strongly refractile colorless body 

 surrounded by a limiting membrane (Fig. 76), which The bodies of the bacilli 

 can be separately stained and may remain within are stalned with meth y- 



,, ,, , f . if-, -, j- lene blue, the spores with 



the cell membrane or may free itself by degeneration fuchsin. 

 of the latter. Endogenous spore formation is com- 

 mon in bacilli, rare in spirilla and in cocci. The spores appear to be 

 surrounded by a dense envelope, and are, as a rule, much more resistant 

 to deleterious agencies, such as heat, drying, chemicals, etc., than are 

 the vegetative forms of the bacteria themselves. 



Vacuoles in bacteria are often mistaken for spores. Spores, when 

 placed under favorable conditions in the presence of moisture and nutri- 

 ment, swell, become less refractile, and develop into the usual vegetative 

 form. The actual observation of this transformation is, in doubtful 



