CHAPTER II 

 THE TISSUE SYSTEMS 



The cells in higher plants are generally grouped into well-marked 

 systems of elements which are known as the tissues. Tissues are 

 sometimes denned as aggregations of cells performing a similar 

 function. This definition is, however, open to some objections, as 

 is also that which describes a tissue as a mass of cells of similar 

 origin. Characterization of tissues, either by their functions or 

 by their mode of origin, seems less desirable than a definition which 

 makes clear that the aggregations of cells have a common organiza- 

 tion. From the standpoint of evolution it is the structural features 

 of the tissues which are of the greatest significance. In works 

 dealing with so-called physiological plant anatomy the functions 

 of the tissues rather than their peculiarities of structure are 

 naturally most emphasized. From the point of view of the doc- 

 trine of descent functional features are of less significance, since 

 it is precisely these which are the most readily modified and as a 

 consequence furnish the least valuable indications of the course 

 of evolutionary development in any given large group. Because 

 the present work deals with anatomy from the outlook of evolution, 

 structural organization in the case of the tissues stands in the fore- 

 ground, although, of course, the question of the functioning of the 

 various tissue systems cannot be left out of view. 



It is an interesting general fact that the boundaries between the 

 tissue systems are much more marked in the plants which are 

 geologically older and lower in the scale of evolution than they are 

 in the higher seed plants, the conifers, and the angiosperms. 

 Further, the more conservative organs of the higher forms namely, 

 the root and leaf exemplify a sharper delimitation of the tissue 

 systems than does that most progressive of all plant parts, the stem. 



For the purpose of the present work the tissues of plants may 

 be divided into three distinct systems, which can be most easily 

 identified by reference to the accompanying figure (Fig. 6) of a 



