90 BOTANT. 



is clear that systems of tissue 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 existence in the youngest parts of plants, but that 

 they result from a subsequent development. 



115. Many systems of tissue 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. The three systems proposed 

 by Sachs are instructive, and will be followed here ; they 

 are : (1) the Fundamental System, which includes the mass 

 of unmodified or slightly modified tissues found in greater or 

 less abundance in all plants (except the lowest) ; (2) the 

 Epidermal System, composed mainly of the boundary cells 

 and their appendages (hairs, scales, stomata, etc.) ; (3) the 

 Fibro-vascular System, comprising those varying aggrega- 

 tions of tissues which make up the string-like masses found 

 in the organs of the higher plants. 



II. THE EPIDERMAL SYSTEM OF TISSUES. 



116 This is the simplest tissue system, as it is the ear- 

 liest to make its appearance, in passing from the lower forms 

 to the higher. It is also (in general) the first to appear in 

 the individual development of the plant. It is sometimes 

 scarcely to be separated from the underlying mass, as in 

 most higher Thallophytes and Bryophytes ; and here it is 

 composed of but one tissue parenchyma or of two or more 

 slight variations of it. In Pteridophytes and Phanerogams. 

 while it may be very simple in some (aquatic) plants, it fre- 

 quently attains some degree of complexity, and is sharply 

 separated from the underlying ground tissues. 



117. In the simpler epidermal structures of the Thallo- 

 phytes the cells are generally darker colored, smaller, and 

 more closely approximated than they are in the subjacent 

 mass ; in some higher fungi a boundary tissue may be easily 

 separated as a thickish sheet, but probably in such case a 



