THE MECHANISMS OF ORIENTATION AND GROWTH 191 



a satisfactory quantitative check with Castle's actual figures. Further- 

 more the torque required can be calculated roughly, and the calculated 

 figure again agrees with the observed. 



(3) Finally we come to the most searching test — the explanation of 

 the reversal of spiralling at the renewed onset of growth after the 

 sporangium has developed. The sign of AcfyjAL depends upon the 

 sign of (1— 2«/^) and in left-hand spiraUing is positive since Injq is less 

 than unity. If, however, q sufficiently decreases at any time or if n 

 increases, then Infq might become greater than unity, when (I— Injq) 

 would be negative and the spiralling would therefore reverse. There is 

 therefore here a possibility of explaining reversal, and the particular 

 explanation which has been put forward, which is not the only possi- 

 bility, is this. As new chitin particles are intercalated in the wall their 

 mutual orientation will be improved by the increase in area of the wall. 

 This will maintain both n and ^ at a certain level. Once elongation has 

 ceased, however, further new deposits will no longer undergo this 

 improved orientation. The value of q which is known to be very sus- 

 ceptible to orientation (Table IV), will decrease and of n probably 

 increase so that when growth recommences the value of (1 —Injq) may 

 well be negative. The cell wiU then rotate in a right-hand spiral. Growth 

 will, however, progressively increase the degree of parallel orientation, 

 so that q will progressively increase and n diminish, until the final values 

 are closely similar to those obtaining before elongation ceased. Hence 

 the right-hand spiralling wiU slow down and finally revert to left hand. 



No quantitative check of this latter effect has yet proved possible, 

 and there are other possible explanations of the reversal within the 

 framework of the theory, some of which have been pointed out by 

 Roelofsen (see refs. in (62)). It is worth noting, however, that cessation 

 of growth at any time should cause a reversal if the present theory holds, 

 and it is therefore exceedingly satisfactory that it has recently been shown, 

 again at the hands of Mrs. Middlebrook, that when growth is temporarily 

 stopped in late Stage IV sporangiophores by immersion in very weak 

 detergent solutions then, on the resumption of growth, the growth 

 spiral is first right-handed and then slowly reverts to left-handed, just as 

 it does in the normal phenomena associated with the early Stage IV 

 condition (62) (Fig. 65). 



Finally we should add that a further check has been made that the 

 relation between A<f>jAL and a, the diameter of the sporangiophore, is 

 of the kind predictable from equation (1), p. 189 (62). 



It is therefore possible in this particular case that some stages of 

 growth do involve the tensile properties of the wall, and it seems likely 



