SYSTEMS OF TISSUE CLASSII'H ATION 39 



outstanding schemes of tissue classification. Future progress 

 may be expected when our insight into the developmental and 

 functional potentialities of the various types of cells has been 

 increased. It seems clear that the fields of pathological and ex- 

 perimental plant anatomy are destined to contribute largely to 

 a more natural grouping of cells and cell aggregates. Weber 

 (1929), who has characterized all previous anatomy as "cell wall 

 anatomy," contends that a firm basis for the distinction of cell 

 types and tissues must depend upon a better knowledge of proto- 

 plasm, with less attention to the morphology of dead and fixed cell 

 walls. In his view, this requires the careful observational and 

 experimental investigation of living protoplasts, which may be 

 "physiologically" distinct although "morphologically" identi- 

 cal. This new approach, which Weber terms "Protoplasmic 

 Plant Anatomy, ' ' is still in its infancy, but undoubtedly a better 

 knowledge of structure will appear as our knowledge of the 

 behavior and potentialities of living cells and cell groups increases 

 [cf. the reviews by Bloch (1941) and White (1941)]. 



Since all methods for classifying plant tissues are open to 

 objection, the writer has adopted a non-committal and "practi- 

 cal ' ' attitude in this book. Instead of following any one scheme of 

 classification, the emphasis is placed first of all upon the salient 

 morphological features of the principal types of plant cells. 

 These cell types recur in various regions, "tissues" and organs 

 of the higher plants, and a thorough knowledge of their form, 

 structure, development, and presumable function (s) must con- 

 stitute the necessary analytical approach to anatomy. If such 

 knowledge is gained through practical laboratory studies, the 

 student should be in a position to study with some degree of 

 independence the comparative anatomy of such organs as the 

 stem, root, and leaf. 



In the appended table an effort is made to summarize the 

 important features of the main types of plant cells. Reproduc- 

 tive cells (spores and gametes) as well as specialized secretory 

 or sensitory cells have been deliberately omitted. No pretense of 

 "completeness" is therefore made, but it is hoped that the table 

 may serve its purpose as a basis for the analytical study of plant 

 structure. 



