THE GROUPS OF TISSUES, OB TISSUE-SYSTEMS. 39 



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 

 may bo called Groups or Systems of Tissues. A Tissue- 

 system may then be described as an aggregation of elemen- 

 tary tissues forming a definite portion of the internal 

 structure of the plant. 



63. Prom what has already been said, it is clear that sys- 

 tems of tissues do not exist in the lowest plants, and that 

 they reach their fullest development only in the highest 

 orders. It is evident also that these systems have no ex- 

 istence in the youngest parts of plants, but that they result 

 from a subsequent development. Many systems of tissues 

 might be enumerated and described; but here again, as 

 with the elementary tissues, while there are many varia- 

 tions, there are also many gradations, having on the one 

 hand a tendency to give us a long list of special forms, and 

 on the other to reduce them to one, or at most to two or 

 three. 



64. The three systems proposed by Sachs are instructive, 

 and will be followed here; they are: (1) the Epidermal 

 System, composed mainly of the boundary cells and their 

 appendages (hairs, scales, breathing-pores, etc.); (3) the 

 Fundamental System, which includes the mass of unmodi- 

 fied or slightly modified tissues found in greater or less 

 abundance in all plants (excepting the lowest); (3) the 

 Fibro-vascular (or Skeletal) System, comprising those vary- 

 ing aggregations of tissues which make up the stringlike 

 masses or woody bundles found in the organs of the higher 

 plants. 



65. In the primary meristem at the end of a shoot or 

 root in the highest plants, several differentiations of the 



