CHAPTER VII. 



TISSUE SYSTEMS. 

 I. THE DIFFERENTIATION OF TISSUES INTO SYSTEMS. 



113. It rarely happens that the tissues which compose 

 the body of a plant are uniform. In the 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 environment. Certain inner cells, or lines of cells, be- 

 come modified into sclerenchyma, or some other supporting 

 tissue (collenchyma, or fibrous tissue), and here again there 

 is a manifest relation to the environment of the plant. Cer- 

 tain 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 phy- 

 siological terms, there may be a boundary tissue, a support- 

 ing tissue, and a conducting tissue, lying in the mass of less 

 differentiated ground tissue. 



114. In different groups of plants the elementary tissues 

 described in previous paragraphs (99 to 108) 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 

 aggregated in particular ways in each plant, constituting 

 what may be called Groups or Systems of Tissues. A Tis- 

 sue System may then be described as an aggregation of ele- 

 mentary tissues, forming a definite portion of the internal 

 structure of the plant. From what has already been said, it 





