CARBOHYDRATES, CHITIN AND CUTIN 



305 



not only to travel upwards, but at the same time to rotate around 

 the axis of the sporangiophore (Fig. 149). There is nothing in the 

 submicroscopical texture of the primary wall which might account for 

 this behaviour. Oort and Roelofsen (1932) state that the isolated 

 wall is flabby and flexible and, as it tears impartially in all directions, 

 is not ot parallel texture. This is con- 

 firmed by the electron microscopic 

 evidence. However, if the interior 

 pressure in the zone of growth is 

 artificially enhanced, the membrane 

 bursts through a very steep spiral 

 longitudinal tear, which may be at- 

 tributed to the anisotropic states of 

 tension in all tubular walls described 

 on page 289. Artificial extension of 

 the zone of growth is accompanied 

 bv a rotation which, after relaxation, 

 recovers. Thus the optics point to a 

 woven tubular texture, while the 

 mechanical properties require a spiral 

 texture. Castle (1942) discovered ad- 

 ditional complications; he was able to show that at first there is 

 regularly a left tendency in growth, which then suddenly changes 

 for an hour to a right-hand spiral and then reverts again to a left 

 spiral. He tried to account for this by suggesting the preformation 

 of both a left-hand and right-hand screw in the primary wall; that 

 is to say, it would be a crossed system indistinguishable from the 

 tubular texture. Preston (1948) has even developed a formula 

 for calculation of the change of rotation from the elastic properties 

 of the cell wall which alter during its diff'erentiation. But all these 

 considerations are based on a spiral texture (Preston, 1934, 1936) 

 which obviously is not realized in the growth zone of the Phjcomyces 

 sporangiophore (Roelofsen, 1949/50, 1951a). Therefore, the simplest 

 assumption is that intercalary growth in the zone of extension travels 

 in a circle; this must be so, since the slender conical shape of the zone 

 of growth could hardly be maintained if the surface grew simul- 

 taneously on all sides. In fine with this is the fact observed by Oort 

 and Roelofsen, viz., that in Phjcomjces Blakeskeanns var. piloboides 



Fig. 149. spiral growth of Phycomyces 



(from Castle, 1937 a). Zone of 



growth dotted; • marks to trace 



rotation. 



