CHAPTER 3 

 GROWTH 



Growth may be considered either as an increase in cell number or as an 

 increase in mass. Usually both these processes are concurrent in the 

 phenomenon called growth. To a limited degree, fungus cells may 

 divide and form new cells without an increase in mass. A spore may 

 germinate in distilled water and give rise to a germ tube, but in the 

 absence of nutrients this process soon stops. A few cell divisions exhaust 

 the reserve material originally present in the spore, and growth soon 

 ceases unless these new cells obtain nutrients from the external environ- 

 ment. Under certain conditions fungus cells may increase their store of 

 reserve materials, and thus their mass, without an increase in cell number, 

 but this process is also limited. Growth, excluding the limited meanings 

 given above, involves an increase in both the number and the mass of cells. 



This definition of growth neither ''explains" the processes involved nor 

 indicates their complexity. Rahn (1932) has expressed doubt that we 

 will ever fully understand the process of growing. A yeast cell which 

 buds and produces a daughter cell illustrates one of the striking features 

 of growth : growth involves duplication. From a dozen or so simple chemi- 

 cal substances present in the medium the parent cell synthesizes at least 

 a portion of the protoplasm of the daughter cell. The daughter cell has 

 the same genetic constitution as the parent cell, and thus a duplication of 

 genes is a feature of cell multiplication. The compounds which comprise 

 protoplasm, enzymes, genes, and other substances are extraordinarily 

 complex. Our meager knowledge concerning the chemical architecture 

 of these substances only confirms this view. In the synthesis of such 

 compounds we may assume that the chemical reactions which produce 

 them are perfectly timed and coordinated, for no series of uncorrelated 

 reactions could produce such compounds. 



The growth processes of the filamentous fungi are still more complex 

 than those of yeast, because of greater differentiation in structure. In 

 those species of fungi which produce aerial mycelium these parts are 

 nourished through the mycelium in contact with the medium. This 

 involves translocation of nutrients over considerable distances. This is 

 especially true of sporangiophores and aerial fruit bodies. The develop- 

 ment of fruiting structures and spores is growth, in that the formation of 

 new cells is involved. The formation of fruit bodies in many species 



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