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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, however, 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 and 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 
