32 RELATION OF PHYLLOTAXIS TO MECHANICAL LAWS. 



by Wildeman * to really support it, so far, that is to say, as an angle 

 of 90° can be judged by the eye in a cleared preparation ; and the 

 law of the orthogonal intersection of cell-walls at a growing apex 

 may be taken as generally accepted. 



The most remarkable deduction from this theory of orthogonal 

 cell-formation is that the cells at the initial group in Zone I. are 

 laid down in accordance with definite mechanical laws, and not as 

 the expression of any aim, on the part of the protoplasm as to their 

 ultimate use in the plant economy. The fact that hexagonal 

 packing appears to be the more frequent case in mature organs is 

 therefore due to secondary alterations in the arrangement, and is 

 the expression, that is to say, of secondary forces initiated away 

 from the growing point itself. In other words, plants form their 

 cells fairly isodiametric and orthogonal in Zone I. ; in the second 

 zone elongation and further orthogonal division takes place ; while 

 a third zone of differentiation is necessary to correct the errors of 

 Zone I. and convert the mechanically produced cells into units more 

 suited for the performance of their special functions. The unequal 

 tensions set up in this process may result in the rolling of the 

 cells over each other as they tend to round off and become more 

 independent. 



On the other hand, it is quite true that the orthogonal position is 

 very difficult to prove absolutely in any particular cell. Angles 

 may look very like right angles without being exactly 90°;. for 

 example, it is often difficult in a transverse section of the apical cell 

 of Pteris to say whether the curvilinear angles are 90° or nearer 

 120°. It is, in fact, only by bringing the great mass of facts into 

 line with some general mechanical principle, as Errera has done, 

 that the probability becomes practical proof. 



Thus when the large oospheres of Fucus or the tetraspores of one 

 of the Florideae are discharged, they assume an apparently per- 

 fectly spherical form, although this is not the shape in which they 

 are actually developed. But regarded as masses of a dense fluid 

 freely suspended in a non-miscible medium of approximately 

 equal density, the surface tension is sufficient to reduce their surface 

 to a minimum, and spherical form is attained by these reproductive 

 * Wildeman, Etudes swr V attache des cloisons eellulaires, 1893. 



