EPIDERMAL SYSTEM 47 



63. Three tissue systems are easily recognizable in 

 the higher plants apart from the less differentiated mass 

 of cells in which they lie. These are: (1) the epidermal 

 system, composed mainly of the boundary cells and their 

 appendages (hairs, scales, stomata, etc.) ; (2) the conducting 

 system, comprising those tissues which are water or 

 food conducting and the tissues immediately associated 

 with these; and (3) the mechanical or skeletal system, 

 consisting of the fibrous tissue, collenchyma and scler- 

 enchyma which furnish the rigidity and strength 

 necessary for the plant. The latter two are sometimes 

 considered together as the fibrovascular system, while 

 the remaining tissues are often grouped under the name 

 fundamental system. The latter is, ho'wever, no definite 

 aggregation of tissues but rather the residue of less 

 strongly specialized tissues from which we have rather 

 arbitrarily set off the other tissue systems, for we must 

 remember that these are all coherent parts of one plant 

 body and not separate parts without close interrelation. 



64, The Epidermal System of Tissues. This is 

 perhaps the earliest tissue system to have been differ- 

 entiated from the remainder of the plant. In many 

 lower plants, the exterior and interior cells show no 

 visible differences, but even here among some we 

 find that the outer cells are more closely crowded together 

 and smaller while the inner cells are loosely arranged. 

 In the fruits of some fungi, the outer layers of cells are 

 firm and resistant. Some of the Liverworts arid Mosses 

 possess an outer layer of cells distinct from the inner 

 cells and evidently of protective nature. It is only in 

 the higher, more massive land plants, however, that we 

 find a really distinct epidermal system of tissues. Thus 

 in the Ferns and onward through the various Fern Allies 

 and throughout the Seed Plants, the epidermis and its 



