THE MECHANISMS OF ORIENTATION AND GROWTH 189 



chains in the growth zone and to make at least some estimate of the elas- 

 tic properties of the wall in that region. Both have now been achieved. 



From the time of the first publication in 1931 by Oort and Roelofsen, 

 it was known that the chitin chains in the growth zone are oriented 

 almost transversely and, although this has since been questioned from 

 time to time, there can now be no doubt of its general validity. The 

 earher observations were made solely under the polarization micro- 

 scope and such observations with chitin are difficult to interpret 

 unambiguously both on account of the low birefringence of the walls 

 and of the sensitivity of the sign of the birefringence to the nature of 

 the medium in which the material is immersed for observation. The 

 case clearly called for analysis by X-ray methods. This, however, 

 involves the careful drying of many sporangiophores which must then 

 be lifted from the glass on which they are attached and piled one on 

 another, keeping the growth zones coincident and in careful parallel 

 alignment, until a bundle of least 0-5 mm. thick is obtained. Unfor- 

 tunately these sporangiophores tend to distort badly if dried without 

 some considerable care, and, when dried, are extremely fragile. The 

 author is therefore fortunate indeed that the dehcate manipulative skill 

 of Mrs. Middlebrook, working in his laboratory, has buik up a suitable 

 bundle, the X-ray diagram of which shows that the chitin chains 

 do run almost transversely. Following this, observation under the 

 polarizing microscope by the methods described on p. 61 et seq. (with the 

 use of an arc lamp in place of the normal microscope lamp on account 

 of the low birefringence) has shown that in Stage IV sporangiophores 

 the chains make an angle to the transverse of the order of 14°.* With 

 this information we can proceed to consider an interpretation of 

 spiral growth which was, in fact, put forward (60) before the structure of 

 the wall was known with certainty. It is not possible here to go into 

 any detail so that only a brief summary will be given. Further informa- 

 tion can be obtained from the original papers (60, 62). 



The wall in the growth zone consists, in effect, of a number of flat 

 spirals. Now we know that when a flat spiral spring is extended so that 

 one end is free, then this end twists. In a spring of radius a in which the 

 winding is circular in section and makes an angle a to the transverse 

 (Fig. 64) then if the spring is loaded axially as shown the rotation of 

 the end, A<i>, for an elongation of AL is given by the relation, 



Aj> cos a sin a (I— 2n I q) 



AL a[cos^ a-l-ilnlq) sin^ a] ' 



.(1) 



♦Considerable care has to be exercised here, since chitin possesses negative 

 intrinsic birefringence coupled with (usually overriding) positive form birefringence. 



