N 



108 THE QUANTITATIVE METHOD IN BIOLOGY 



plants have ordinarily a regular position when elongated 

 and are rather in disorder when isodiametric. 



It happens that the limits between the segments of a chess- 

 board system are obsolete, each segment being represented by a 

 vexDlary mark (§ 8i). If disorder exists at the same time, the 

 chess-board-like disposition may be at first sight indiscernible 

 because of the irregular position of the marks. EXAMPLE : 

 Disorder often prevails in the disposition of the hairs with 



which the epidermis of 

 many insects and plants 

 is clothed, each hair 

 representing one seg- 

 ment. 



The plan of structure 

 is, however, often recog- 

 nizable by comparing 

 several species (some of 

 them being regular), or 

 by investigating accur- 

 ately the object in order 

 to discover traces of the 

 limits, and perhaps by 

 studying its embryonal 

 development. 



Disorder is certainly 

 the most important 

 cause owing to which 

 the universal distribu- 

 tion of the biaxial system 

 in animals and plants 

 has been hitherto over- 

 looked. I regret that I 

 have not been able to 

 find any method to 

 Fig. i6. — Mnium punctatum. Cells of the leaf- measure the degree of 



N-N, nerve. Ad naturam disorder. 



N 



§ 86.— ALTERATIONS OF THE CHESS-BOARD SYSTEM 

 {continued). OBLIQUE AXES IN THE PLANE ISIS-EW 

 (FALSE AXES ?).— In a number of biaxial systems the dis- 

 position of the segments points to the existence of oblique axes. 

 This is observed, for instance, in the leaves of certain mosses. 

 In Mnium punctatum the cells are often placed in longitudinal 

 and oblique rows, the latter being directed from the nerve 

 towards the margin and from the base towards the summit 

 of the leaf (Fig. i6). This is described as a character of the 

 species. 



