SYSTEMS OF TISSUE CLASSIFICATION 37 



ever, many tissues or cell types carry on more than a single func- 

 tion. In such instances, a distinction between "principal" and 

 "subsidiary" function appears somewhat arbitrary. In other 

 words, certain types of cells might with equal justification be 

 classed in more than one of the anatomico-physiological "sys- 

 tems." Furthermore, as Luiidegardh admits, the principal func- 

 tion of a tissue (e.g., storage of reserve starch) can only rarely 

 be deduced from observation only. Hence, any anatomico-physio- 

 logical classification has a provisional character and is destined to 

 be changed or modified in the light of new experimental data. 



In the second place, the objection is raised that in such a 

 scheme as Ilaberlandt proposes, confusion results because of the 

 disregard of the origin of cells and tissues. For example, in 

 Ilaberlandt 's classification, epidermal and cork cells, although 

 differing fundamentally in origin, are grouped for physio-topo- 

 graphical reasons under the "Dermal" or protective system. 

 Conversely, guard cells and root hairs, while having a common 

 origin from the embryonic surface cell-layer or "protoderm," 

 are classified because of functional differences in the "Ventilat- 

 ing ' ' and ' ' Absorbing ' ' systems respectively. In short, as Ilaber- 

 landt (op. cit., p. 70) emphasizes, to the physiological anatomist 

 "the homologies of tissues are of no interest . . ." in defining 

 and classifying the various tissues of the plant body; "... his 

 concern is solely with analogy. " Whether such a viewpoint leads 

 to a "natural" insight into the evolutionary development of plant 

 tissues is open to serious question. Jeffrey (1917, p. 8) says in 

 this regard that "from the point of view of the doctrine of 

 descent, functional features are of less significance, since it is 

 precisely these which are the most readily modified and as a con- 

 sequence furnish the least valuable indications of the course of 

 evolutionary development in any given large group." 



3. Fames' and MacDaniels' classification. In contrast to the 

 schemes of Sachs and Ilaberlandt, Fames and MacDaniels base 

 their classification of tissues on method of development. From 

 this standpoint, tissues "whicli are developed directly or indi- 

 rectly at the growing points by cell division in several or many 

 planes" are termed })rimary tissues. On the other hand, tissues 

 which "are formed largely by cell division in a single plane. 



