THE MECHANISMS OF ORIENTATION AND GROWTH 187 



mentioned in Chapter VI. In Hydrodictyon, the smallest cell examined 

 possesses a wall in which the cellulose chains are oriented at random, 

 and no matter how long the cells become the orientation remains 

 random, so that this elongation is presumably associated with some 

 factor other than tension due to turgor forces. In another sense, the 

 existence of the two algae Cladophora and Spongomorpha with the same 

 growth form but vastly different wall structure, points in the same direc- 

 tion. Nevertheless some caution is to be exercised, for there are isolated 

 cases in which growth phenomena do seem to involve tensile effects in 

 the walls. 



No more striking example of these latter cases can be taken than the 

 growth of the sporangiophores of Phycomyces as first investigated by 

 Oort and Roelofsen and later by Castle (59). Thanks largely to the 

 meticulous work of this latter investigator, a great deal is now known 

 of this matter and only the gist of it can be given here. 



Spiral growth in the sporangiophores of Phycomyces 



The sporangiophore grows towards the light from the substrate as a 

 thin cylinder, whose wall is composed of chitin impregnated with protein 

 and other substances. The peculiarity which has led to so much effort 

 being put into its investigation is that during its growth period it not 

 only elongates but also twists around its own axis. This twisting, how- 

 ever, is not noticeable unless markers, in the form of light glass fibres or 

 Lycopodium spores, are placed upon the tip of the sporangiophore and 

 observed over a period of time. Growth occurs only in the apical 2 mm. 

 or so of the sporangiophore, where the wall remains thin and may be 

 called a primary wall. Below this growth zone, secondary wall de- 

 position effectively prevents any further elongation. The progressive 

 elongation of the growth zone, and the continual removal from it of 

 its lower section by deposition of these secondary layers, maintains the 

 depth of the zone more or less constant. There is no morphological 

 twist such as may be seen in, for instance, bindweed twining round a 

 support: the twist is achieved through displacements of the wall 

 substance at the molecular level. 



At this period, before the sporangium has begun to appear, the cell 

 twists left handed, i.e. looking down on the tip of the sporangiophore, 

 a marker will move in a clockwise direction. The onset of sweUing 

 in the sporangium calls a halt both to elongation and twisting and this 

 marks the end of what is called Stage I. Once the sporangium is fully 

 swollen (Stage II), the whole structure rests for a time (Stage III) and 

 then once again begins to elongate and to rotate. Now, however, the 



