i 5 8 RESEARCHES ON FUNGI 



in size and the vacuoles which had increased in size began to decrease 

 in size. Whether or not in general in Rhizopus nigricans and other 

 Mucorineae the rhythmic backward and forward flow of protoplasm 

 is normally associated with a rhythmic waxing and waning of vacu- 

 oles remains to be determined by further more detailed observations. 



The Biological Significance of Septa in the Mycelium of the Higher 

 Fungi. — The cylindrical wall which bounds the hyphae in the 

 Phycomycetes, Ascomycetes, Basidiomycetes, and Fungi Imper- 

 fecti has several functions : (1) it serves as a sort of exoskeleton in 

 that it supports the protoplasm and maintains the form of the 

 hyphae ; (2) it protects the protoplasm from small mechanical 

 injuries ; and (3) as a firm elastic membrane it opposes the outward 

 pressure of the cell-sap and thus permits hyphae to become turgid. 



The j^ounger mycelia of the Phycomycetes are non-septate ; 

 whereas, in the mycelia of the Ascomycetes, Basidiomycetes, and 

 Fungi Imperfecti, septa are developed from the first. In these 

 septate mycelia each septum is a thin flat circular plate with a 

 relatively small central open pore (cf. Figs. 67 and 78, D, pp. 131 

 and 154). In most septate hyphae the septa are situated at con- 

 siderable distances from one another. Thus in some leading hyphae 

 of Pyronema confluens, with a diameter of about 8 /x and having 

 thirty-two septa, the average distance between two successive septa 

 was 1 33 jm, so that the cells were about seventeen times as long as they 

 were thick (cf. Fig. 63, C and D, p. 116). Taking into account the 

 peculiar structure and the location of septa we may ask : what is 

 the biological significance of a septum or, in other words, how do 

 septa promote the welfare of the living hyphae in which they are 

 situated ? An attempt to answer this question will now be made. 



Septa must strengthen hyphae mechanically by increasing the 

 resistance to bending or breaking where they are situated ; but, as 

 the cells of septate hyphae are often 10-20 times as long as they are 

 wide, the mechanical advantage accruing to hyphae through being 

 septate must on the whole be but slight. Evidently we must look 

 in another direction for the chief biological significance of septa. 



In all fungi, whether high or low, the protoplasm in a hypha is, 

 normally, a continuous mass. In a hypha of one of the Phycomy- 

 cetes the mass of protoplasm is more or less cylindrical ; whilst, in 



