GROUPS OF TISSUES, OR TISSUH-SYSTEMS. 83 
may be 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. 
61. From 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. 
62. Many systems of tissues might be enumerated and 
described; but here again, as with the elementary tissues, 
while there are many variations, there are also many grada- 
tions, 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. 
63. 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 ap- 
pendages (hairs, scales, breathing-pores, etc.); (2) the Fun- 
damental System, which includes the mass of unmodified or 
slightly modified tissues found in greater or less abundance 
in all plants (excepting the lowest); (3) the Fibro-vascular 
System, comprising those varying aggregations of tissues 
which make up the string-like masses or woody bundles 
found in the organs of the higher plants. 
64. The Epidermal System of Tissues.—This is the sim- 
plest tissue-system, as it is the earliest to make its appear- 
ance, in passing from the lower forms to the higher. It is 
also (in general) the first to appear in the individual devel- 
opment of the plant. It is sometimes scarcely to be sepa- 
rated from-the underlying mass, as in most lower plants; 
