DISEASES CAUSED BY FUNGI 35 



SCHIZOMYCETES, BACTERIA. 



Bacteria are exceedingly minute cells, enclosed in a cell 

 wall and frequently furnished externally with protoplasmic 

 threads which serve for locomotion. Some species form an 

 internal resistant body (endospore), which enables them to 

 withstand desiccation and in some cases exposure for a consider- 

 able period to the temperature of boiling water. Reproduction 

 is by division of the parent cell into halves, a method quite 

 distinct from the budding of the yeasts, which are true fungi. 



Bacteria are classified into genera by form, and into species 

 mainly by their behaviour in cultures on various media. Their 

 part in the causation of plant disease is discussed in Chapter II. 

 Examples of bacterial diseases will be found under cotton, 

 tomato, and citrus. 



EuMYCETES, Fungi. 



The characteristics of fungi have already been described in 

 general terms. The vegetative body (thallus) of a fungus 

 typically consists of branching filaments (hyphse), growing at 

 the tip, free or combined into more or less definite structures, 

 but may consist of rounded cells free or in groups, multiplied 

 by budding, as is the rule in the yeasts, and is the case in certain 

 stages or conditions of normally filamentous fungi. 



Classification is based on the nature and form of the repro- 

 ductive organs and spore-bearing structures. Where as is often 

 the case the same fungus has more than one method of spore 

 formation, the direct production of conidia from hyphae, free or 

 enclosed, is regarded as subsidiary to spore production by way 

 of asci or basidia, and any generic name based on the former is 

 superseded by the one appropriate to the latter. Thus Colletotri- 

 chum gossypii became Glomerella gossypii when the fungus was 

 found to possess an ascogenous fructification, though in this 

 as in many instances it is convenient to retain in common use 

 the name appropriate to the condition of the fungus as it is 

 ordinarily found. 



Class I : Phycomycetes. 



These are fungi which retain considerable resemblance to 

 certain groups of algge, from which they are thought to have been 

 derived, including a distinct provision of structures for sexual 

 reproduction. The latter have in some cases lost their function 

 and are often largely superseded by the formation of asexual 

 spores. The mycelium, which is commonly well-developed and 

 freely-branched, is without septa except in old hyphse or in 

 connection with reproductive structures. There are two sub- 

 classes : 

 I. Oomycetes. 



Most of the species form oospores as a result of a perfectly 

 or imperfectly retained process of the fertilisation of an egg-cell 



