The Plant Cell 17 



" the important substratum of all vital activity." Refer- 

 ring to the cell-theory and the importance of it, Wilson 

 concludes: " No other biological generalization, save only 

 the theory of organic evolution, has brought so many ap- 

 parently diverse phenomena under a common point of view 

 or has accomplished more for the unification of knowledge. 

 The cell-theory must therefore be placed beside the evolu- 

 tion-theory as one of the foundation stones of modern 

 biology." 



10. Early use of the term " cell." In the earlier studies 

 upon the cell, beginning in the latter part of the seven- 

 teenth century, the term was applied to the firm walls 

 alone, from their resemblance to the cells of the honey-comb. 

 When, however, protoplasm, or the living substance within, 

 was later discovered, and its significance as the important 

 morphological and physiological unit -determined, the 

 same term was retained for this essential unit of living 

 substance. Nevertheless, with the obvious distinction in 

 mind, the term is still applied to the many cell-forms or 

 cell-cavities, from which all living matter has disappeared, 

 such cell-forms of many types constituting the great 

 bulk of the conductive tissues of woody plants, and all 

 of the heart-wood, stony tissue, dry bark, and the like. 



11. Meristem or embryonic cells. Structurally or 

 physiologically the term "cell" is now employed to denote 

 the simplest unit into which the organism may be con- 

 veniently resolved. It consists essentially of a unit mass 

 of living protoplasm with certain inclusions or surround- 

 ing materials. 



In plants the protoplasm is usually inclosed by firm 

 often box-like cell-walls. Plant cells are usually so diverse 



