Exercise IV 



PROBLEMS IN THE CLASSIFICATION OF CELL TYPES, 

 TISSUES, AND TISSUE SYSTEMS IN VASCULAR 



PLANTS 



I. Introduction. — Despite the vast amount of information 

 which has aeeiimiilated during the present century regarding- the 

 cellular structure of higher plants, the nomenclature and classi- 

 fication of cell types and "tissues" is still in a confused and 

 uncertain state. Doubtless much of the difficulty arises from 

 the absence in plants of the high degree of structural and physio- 

 logical individuality which characterizes the tissues of higher 

 animals. For example, certain so-called ' ' permanent " or " adult ' ' 

 tissues, such as parenchyma, collenehyma, and epidermis, may 

 "secondarily" revert to a meristematic state and produce tissues 

 or structures quite different from themselves. Furthermore, 

 it is difficult or even impossible to draw a clear morphological 

 demarcation between adjacent "])ermanent" tissues in many 

 instances. A good example is furnished by the gradual inter- 

 gradation between collenehyma' and parenchyma tissue in the 

 cortex of many stems. As a consequence of the.se and other 

 difficulties, the term "tissue" in Plant Histology has been used, 

 sometimes in a broad sense, sometimes in a restricted sense, 

 (lei)ending upon the relative importance attributed to position, 

 origin, structure, or function. Sachs (1875, p. 68) adopted in 

 the first place a broad concept by stating that "in the widest 

 sense every aggregate of cells which obeys a common law of 

 growth (usually, however, not luiiform in its action) may be 

 termed a tissue." A similar idea is found in Strasburger's Text- 

 book (1921, ]). 41), where a tissue is defined as a "continuous 

 aggregation of cells in intimate union." Other definitions of 

 "tissue" are less general in character and introduce, in various 

 ways, the idea's of origin, specific cell structure, and function. 

 For example, Eames and IMacDaniels (1925, p. 50) define a tissue 



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