CLASSIFICATION OF THE PRIMORDIA 111 



(3) The plane EW-ZN determined by the axes EW and ZN 

 (transverse plane). 



EXAMPLE : In a number of Phanerogams (AsclepiadacecB, 

 ApocynacecB, etc.) the epidermis consists of two layers of cells 

 and is therefore a very simple triaxial system. If we take, for 

 instance, in a leaf the plane which is parallel to the surface of 

 the epidermis as NS-EW, a longitudinal section (parallel to the 

 middle nerve and perpendicular to the epidermis) represents 

 the plane NS-ZN, and a transverse section (perpendicu- 

 lar to the middle nerve and therefore to the surface) repre- 

 sents the plane EW-ZN. When we take the plane NS-EW 

 (parallel to the surface) as base, in each of the superposed 

 systems (two layers), which follow each other in the direction 

 ZN, we may find all the alterations and properties which may 

 exist in any biaxial system (§§ 77-87). Since the superposed 

 layers are more or less independent of each other, differences 

 may be observed between them. The whole triaxial system 

 may be investigated by starting from one of the three planes, 

 for instance, NS-EW, and comparing the layers which are 

 superposed according to the axis ZN, by studying and measur- 

 ing their primordia on sections exactly parallel to the planes 

 NS-ZN and EW-ZN (longitudinal and transverse sections). 



In the triaxial systems the alterations and the primordia of 

 each layer (biaxial system), combined with the differences 

 between the layers, bring about an endless diversity and 

 various complications, which reach the highest degree in the 

 massive parts of the higher animals and plants. ^ 



By the application of the above principles it is theoretically 

 possible to analyse the most difficult objects and to express 

 their properties by measurement. In many cases, however, 

 the structure is so intricate that the difficulties seem to be 

 practically insurmountable as long as the quantitative method 

 is not better established. The impossibility of making sections 

 of one object according to three different planes is a serious 

 obstacle. Therefore, when we need to compare quantitatively 

 several specimens, a choice between the three planes ought to 

 be made, the properties being measured in the same plane in all 

 the specimens. 



A large amount of information has already been collected, 

 especially by histologists and embryologists, about the internal 

 structure of animals and plants. Unfortunately the observed 

 facts have been rarely measured in a convenient way (if 

 measured at all), and the fundamental distinction between 

 simple and compound properties has been overlooked. Much 

 attention has been paid to morphological homologies, but 



1 Complication is often increased by obliquity (see § 86) and also by the 

 curvature of certain axes (see § 84) and by disorder (§ 85). 



