WALL STRUCTURE IN THICK CELL WALLS 99 



tube inserted through the wall, washed out several times with distilled 

 water, inflated with air and dried. A series of circles was then marked 

 on the cell by mounting it on a rotatable spindle and holding a stylus 

 against the wall surface while the cell was rotated. The circles were 

 duplicated on a model of the cell some ten times the size of the cell 

 itself. Now the cell was mounted in the X-ray spectrometer so that any 

 point of its surface could be brought into contact with the slit and be 

 held there by a system carrying two parallel wires; these could be 

 arranged parallel to the tangent to the circles at the point of contact 

 with the slit. In this way a photograph of the piece of wall touching 

 the slit carried also the shadows of the wires which were parallel to the 

 circles. The directions of the cellulose chains could thus be marked 

 both on the specimen and on the model. Now observation of the 

 striations had already shown them to be sensibly straight over at 

 least 2 mm. Hence one of the cellulose chain directions was con- 

 tinued over a distance of li mm. and another photograph obtained. 

 Repetition of this operation enabled a complete survey of one set of 

 chains to be made over the whole wall surface. 



The first set of chains to be followed appeared to make a great circle 

 round the cell, passing over the base of the cell to the tip and back to 

 the base. In some regions, and particularly at the base and tip, the 

 photograph was very diff'use and it was, in fact, difficult to be sure that 

 the chains did run back through the initial starting point — whether 

 we had traversed a single great circle or only part of a flat spiral. 

 Returning to the starting point, therefore, the second set was followed. 

 Here the path was most certainly spiral, and the spiral was laboriously 

 followed round and round the cell for nearly three years. As the spiral 

 closed in more and more towards the base, the photograph became 

 more and more diff'use and it became progressively clearer that a point 

 was being approached at which the typical "crossed" photograph 

 would not be obtained. This point was at last reached (Plate IV, Fig. 2). 

 The model thus finally presented the appearance shown in Plate IV, 

 Fig. 3. Clearly one set of chains forms a slow left-hand spiral round the 

 cell which closes in at the tip and the base. The other set equally 

 obviously forms a series of meridians, uniting the two "poles" of the 

 spiral. It is clear, therefore, why the photograph becomes diff'use near 

 these poles and why, at the poles themselves, it shows a series of circles 

 instead of the usual crossed lines of arcs. 



In view of what we shall find later in other algae, it is interesting to 

 notice that the spiral set of chains in the Valonia wall follows the 

 path of the so-caUed equiangular spiral. The correspondence is not 



