632 PATTERNS AND PROBLEMS OF DEVELOPMENT 



parallel orientation, those of any one layer being inclined in direction to 

 those of the layer adjoining on each side at an angle averaging somewhat 

 less than a right angle. The chains of one set of layers constitute merid- 

 ional systems with reference to the two poles of the cell; those of alter- 

 nate sets, spiral systems centering at the same two poles. In short, the 

 X-ray cellulose pattern shows a definite relation to a cell polarity of some 

 sort.'-' The Fa/o;wa cell exhibits physiological polarity : small uninucleate 

 cells develop from the multinucleate protoplast in one polar region and 

 form holdfasts or rhizoids, and cells giving rise to thallus buds form from 

 the opposite polar region. This polar difference in behavior is similar to 

 that in many plants and animals in which a polar gradient has been 

 demonstrated; consequently, it is probable that a longitudinal gradient 

 system is present in Valonia. At any rate, the cellulose pattern 

 of the cell wall appears to be definitely related to, and probably deter- 

 mined by, a general protoplasmic pattern. Granting that such a relation 

 exists, it does not account for the periodicity represented by successive 

 layers or for the alternation of meridional and spiral orientations of cellu- 

 lose chains constituting the layers. 



In cell walls of multicellular plants, as far as examined, the cellulose 

 micells or crystallites are, in general, parallel to the surface of the wall 

 but may be parallel or transverse to the long axis of the plant or un- 

 oriented. They are predominantly transverse in parenchyma cells of the 

 Avena coleoptile, according to the optical evidence; but a change toward 

 longitudinal orientation occurs when the cell wall is stretched longitudi- 

 nally, though not when growth elongation by action of auxin takes place. 

 In cell walls of the coleoptile epidermis the orientation is predominantly 

 longitudinal and on longitudinal stretching becomes more completely so.''* 

 Apparently cellulose orientations in these cells are determined by local 

 intracellular conditions and relations to adjoining cells. Experimental 

 work on effects of stretching, swelling, etc., on cellophane and plant fibers, 

 as well as on cell walls, indicates that mechanical factors play an im- 

 portant part in orientation. 



Evidence of orientation of particles is also found in chitinous cuticles 

 and exoskeletons of various animals. An orientation of heteropolar mole- 

 cules vertical to surfaces or phase boundaries is another effect of local 

 factors in protoplasms. It does not appear, however, that the microscopic 

 or ultramicroscopic patterns of cell walls or protoplasmic surfaces consti- 



■^ Preston and Astbury, 1937, and their citations of earlier literature. 



'•• See Bonner, 1935, and his citations; also, for general discussion, Seifriz, 1936, chap. xv. 



