94 SPIRAL TWIST A^W OFT. ACTIVITY 



Castle (1936) has recently undertaken the study of the 

 mechanism of spiral growth in Phy corny ces. He claims 

 that the spiral structure of the growing cell wall is not 

 strictly predetermined but depends on the interaction of 

 forces which exert their action in the growth region. The 

 twist of the growing elastic elements of the wall may be 

 the result of their resistance to turgor. Castle construct- 

 ed a model to illustrate this process. If the elastic ele- 

 ments of the wall are distributed symmetrically, dextral 

 spirals ^\i\\ be obtained in 50 per cent of the cases and sin- 

 istral spirals in the other 50 per cent. As the left direc- 

 tion of the spirals is typical for Phycomyces, one must 

 assume a dissymmetric distribution of the elastic ele- 

 ments of the cell wall, which later, under the action of 

 turgor, lead to the formation of sinistral spirals. 



As Castle himself points out, the mechanism of spiral 

 growth can be different in different organisms, and one 

 cannot directly transfer his explanation of the mechanism 

 of twisting to the bacteria, the more so since the cell wall 

 of the latter is thought to consist of some specific pro- 

 tein material closely related by its nature and origin to 

 cellular protoplasm (John-Brooks, 1930). 



But one can assume that in the spiral growth of the cell 

 wall of a bacterium, as in that of Phycomyces, tw^o fac- 

 tors are involved: (1) Some pre-existing asymmetric 

 system (distribution of the elastic elements of the wall 

 in Phycomyces; optically active secondary protoplasmic 

 constituents in bacteria) ; (2) a system of forces directly 

 inducing the spiral twist (turgor in Castle's experi- 

 ments). The interaction of these two factors would bring 

 about the dextrality or sinistrality of the spiral growth 

 according to the following scheme : 



Secondary substance Asymmetric structure Sniral growth 



of metabolism ^ of the cell wall ^ ^ 



f 



Forces directly inducing 

 * the spiral twist 



