166 The Phenomena of Morphogenesis 



cell take up positions at angles of 13.5 or 27° from the long axis of the 

 organ, these angles resulting from the character of the chitin molecule. 

 Denham (1922) observed that in the cotton hair, at least, the spiral 

 markings and striations on the wall coincide with the spiral path of the 

 streaming nucleus and cytoplasm. Preston ( 1948 ) compares the growth of 

 this cell to the pulling out of a flat, spiral spring which rotates as it ex- 

 tends, and he has suggested an explanation for this in mechanical terms; 

 but Castle ( 1936 ) thinks that this does not determine the structure of the 

 growing wall, though it may produce a spiral layering where the wall is 

 not elongating. He points out that not only the structure of the wall but 

 its elastic properties must be taken into account and believes (1953) 

 that, although the growth of the wall is helical, its course is not absolutely 

 fixed by its structure, since the angle of spiral growth can be reversed by 

 a change of temperature. Frey-Wyssling ( 1954 ) calls attention to the fact 

 that in certain polypeptid chains the divergence angles between the 

 amino acid residues show the same regularities as are found in the 

 Schimper-Braun phyllotactic spiral and suggests that the same geometric 

 cause— the necessity for most efficient packing— may underlie both. 



Green ( 1954 ) studied the growth of the long cells of Nitella which had 

 been marked and found that these marks, as well as the two natural 

 striations in the cell, showed a uniform dextral twist. Its regularity is 

 maintained by growth processes evenly distributed through the whole 

 cell and presumably resulting from changes in the fine structure of the 

 cell wall. 



The spiral grain found in the wood of many trees is another manifesta- 

 tion of spirality. This may be very conspicuous in some cases and seems 

 to be most common in trees growing in exposed situations or under 

 unfavorable conditions. The spiral may be right-handed or left-handed. 

 This subject has been reviewed by Champion ( 1925 ) . Preston ( 1949 ) , 

 using the data of Misra (1939), attempted to relate spiral grain to the 

 spiral growth of single cambium initials and assumes that these twist or 

 roll spirally. This would involve some slipping of cells past each other. 

 From what is known of intercellular relationships, it is rather unlikely 

 that such a change occurs. The essential fact in most cases of spiral grain 

 is that vertical files of cells become tilted slightly to the right or to the 

 left and that this results in a spiral course for the cells of the wood. Some 

 slipping of the cells may be involved, but this might be accomplished 

 by localized intrusive growth (p. 82) such as has been shown to take place 

 at the tips of the cambial initials. The tilt seems to be related to a change 

 in cell polarity. Neeff (p. 128) found that when a new polar axis was 

 established the cambial cells gradually changed their direction until this 

 became parallel to the new axis. That there may be a spiral polarity in 

 the trunk itself is suggested by the work of Misra (1943), who reports 



