SYSTEMS OF TISSUE CLASSIFICATION 33 



as "a continuous organized mass of cells, usually similar in oriyin, 

 and essentially alike in form and general function." Hay ward 

 (1938, p. 11) adopts a similar attitude by contending that 

 "strictly defined, a tissue is a group of cells of common origin 

 having essentially the same structure and performing the same 

 functions. " In these definitions, community of origin, continuity 

 and similarity in structure and function are essential attributes 

 of the cells of any "tissue." Ilaberlandt (1914, pp. 56-72), in 

 contrast, has approached the problem from quite a' different point 

 of view. He assumes that the differentiation of specific cell- 

 aggregates or "tissues" in plants is "mainly the outcome of divi- 

 sion of labor, and that consequently the most characteristic 

 features of each tissue are those which are most intimately con- 

 nected with its physiological activity." Haberlandt's concept is 

 thus essentially physiological or functional, and the ontogenetic 

 and phylogenetic aspects of tissues are more or less completely 

 disregarded. 



It is necessary to emphasize that the wide differences in the 

 concepts of "tissues" outlined above have much more than an 

 historical significance. On the contrary, these differences under- 

 lie the varied schemes of tissue classification found in modern 

 botanical texts and continue to influence the study of comparative 

 histology and anatomy. For these reasons, the following critical 

 resume of the most important early as well as recent schemes of 

 tissue classification is offered as a guide for the student in the 

 interpretation and evaluation of modern histological literature. 

 In addition, it is hoped that this critique may serve to emphasize 

 the need for a complete re-examination of the fundamental 

 assumptions upon which these attempted classifications inevitably 

 rest. 



II. Systems of Tissue Classification. — 



1. Sack's classificaiion. Sachs assumed that, in the phylo- 

 genetic development of the higher plants from simple multi- 

 cellular forms, a distinction gradually arose between the outer 

 layers of cells or "tissue" and the internal mass. The latter 

 finally differentiated strands of cells surrounded by "funda- 

 mental" tissue. The final result is seen in the "primary" struc- 

 ture of the leaf, stem, and root, which consist, according to Sachs 



