CHAPTER II 

 THE THALLUS 



The thallus (vegetative body) is naked and at times amoeboid in the 

 simplest families of fungi; in the rest, it is surrounded by a cell wall and 

 is usually in the form of septate hyphae. Under certain conditions of 

 nutrition, as in solutions of small nutritive value, the hyphae grow by 

 sprouting, in which process small protuberances are formed which 

 enlarge, round off and are abjointed (cut off by a septum) from the 

 mother cell, then continue to increase in size and sooner or later separate 

 from the original groups of cells (Fig. 1, a, e,f). These are called sprout 

 cells or occasionally, and less correctly, sprout conidia. In certain 

 groups they form the only type of thallus known. Under unfavorable 

 conditions of growth, the protoplasm contracts, rounds up and secretes 

 a special, thick membrane; these resting cells are called gemmae. Under 

 suitable conditions, they grow to normal thalli. 



In some groups the hyphal wall gives the chitin reaction, in others that 

 of cellulose; in fructifications and resting cells it is usually strengthened 

 by mineral incrustations, by secretions of resins, etc. At first it forms a 

 hyaline membrane which becomes thicker, is further differentiated by 

 secretions and deposits and usually colored by pigment deposits. An 

 unquestioned relation between the fundaments of the wall, especially 

 the septum, and mitosis has been proved only in a few cases; an especially 

 characteristic example occurs in ascospore formation. As a rule the 

 wall is gradually differentiated from the cytoplasm without nuclear aid, 

 in endogenous spore formation simultaneous with cell elongation, in 

 septal formation by furrowing (ring-like thickening of the walls like an 

 iris diaphragm). For the maintenance of intercellular communication, 

 the septa are usually pierced by a few openings through which pass 

 protoplasmic threads 



In rapid growth, septal formation may be delayed, later it is made up 

 for by simultaneous or successive septal formation. In certain groups, as 

 in the Siphonales among the algae, the septa are wholly suppressed; the 

 whole thallus is then a single ramose, multinuclear sac which becomes 

 septate only in the formation of reproductive organs, in conditions of 

 poor nutrition and in age. Since these sacs contain numerous undiffer- 

 entiated energids, they are called coenocytic (polyenergid). 



The individual hyphae usually creep about and are intertwined in 

 felt-like masses. Such a group of hyphae is called the mycelium. In 



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