CHAPTER II. 
THE GROUPS OF TISSUES, OR TISSUE-SYSTEMS. 
58, The Differentiation of Tissues into Systems.—It rarely 
happens that the tissues which compose the body of a plant 
are uniform. Inthe great majority of cases the cells of the 
primary meristem become differently modified, so as to give 
rise to several kinds of tissues. The outer cells of the 
plant become more or less modified into a boundary tissue, 
and the degree of modification has relation to its environ- 
ment. Certain inner cells, or lines of cells, become modi- 
fied into stony tissue, or some other supporting tissue 
(thick-angled, or fibrous tissue), and here again there is a 
manifest relation to the environment of the plant. 
59. Certain other inner cells, or rows of cells, become 
modified into tubes affording a ready means for conduction, 
and appear to have a relation to the physical dissociation 
of the organs of the higher plants, in which only they occur. 
Thus, in physiological terms, there may be a boundary tis- 
sue, a supporting tissue, and a conducting tissue lying in 
the mass of less differentiated ground-tissue. 
60. In different groups of plants the elementary tissues 
described in previous pages are aggregated in different 
ways, and are variously modified to form these bounding, 
supporting, and conducting parts of the plant. Several 
tissues, or varieties-of tissue, are regularly united or aggre- 
gated in particular ways in each plant, constituting what 
