974 THE HIGHER BACTERIA, MOLDS AND FUNGI 



a loose meshwork, without definite arrangement except that certain 

 of them are thrust up vertically and develop spores. In the higher 

 types, of which the mushrooms are familiar examples, certain fila- 

 ments form a cobweb-like net-work which is spread through the 

 soil on which they grow. These serve to absorb and pass on nourish- 

 ment. They connect with other filaments which are welded together 

 to form the firm umbrella-like structure which projects above the 

 ground. This is covered with a tough protective membrane also 

 made up of closely cohering filaments. Along the gills on the under 

 side of the cap are rows of characteristic cells (basidia) on which 

 the spores are born. These different portions are analogous in 

 function to the roots, stems, bark, fruit and seeds of higher plants. 



The gross appearance, the microscopic structure and especially 

 the type of spores produced are relied on for the identification and 

 classification of the various species of fungi. Consequently, it will 

 be necessary to define some of the morphological terms used before 

 proceeding to a description of the different pathogenic types. 



MORPHOLOGICAL DEFINITIONS 



The Thallus. The entire vegetative portion of a fungus is called the 

 thallus; the individual filaments of which it is composed, hyphae. When the 

 hyphae lie in a loose meshwork without definite arrangement, the mass is 

 termed a mycelium and sometimes this term is applied to the entire thallus, 

 even when it develops a characteristic morphology. In some species the 

 hyphae are continuous tubes with multiple nuclei; in others they are divided 

 by septa into chains of cylindrical cells. 



Hyphae which have special functions are often differentiated from the 

 rest of the mycelium. In most of the pathogenic fungi the thallus is rudi- 

 mentary and it is not necessary to discuss here the elaborate and somewhat 

 confused terminology used in 'describing the more complex forms. In the 

 species to be considered only the fertile hyphae, i.e., those that give rise 

 to spores are differentiated. They are called sporophores or conidiopJiores, 

 these terms being applied sometimes to one specialized cell, sometimes to a 

 multicellular or branched filament or to a bundle of filaments. A small cell, 

 or even a conical process from a cell, which serves as a point of attachment 

 for spores is called a sterigma; flask-shaped cells of this type are called 

 phialides. 



Spores. The term spore in the stricter sense means a rounded reproduc- 

 tive cell analogous in function to the multicellular seed of a higher plant. 

 As a rule, a spore differs in form from the parent cell, and does not divide 

 until it becomes separated from the thallus, then after a latent period it 



