TISSUE-SYSTEMS. 



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form the coverings of young stems, roots, and leaves ; 

 (2) The Fibro-vascular System, including such combina- 

 tions as form the stringy masses which abound in the 

 substance of the higher plants; and (3) The Fundamental 

 System, including the combinations of cells which have 

 undergone little or no change of form ; in short, all the 

 rest of the plant except the two systems first mentioned. 

 292. The epidermal system is most highly devel- 

 oped in Phanerogams. Fig. 226 shows a section through 

 the thickness of a leaf. Here 

 it will be observed that there 

 is a closely-packed layer of 

 cells forming the upper sur- 

 face, and a similar layer form- 

 ing the lower surface. These 

 layers constitute the epiderm,is 

 or skin of the leaf. The outer 

 part of the epidermis is usu- 

 ally a continuous layer, and 

 is known as the cuticle. It 

 will be seen that the walls of these- cells are much 

 thicker than those of the cells in the body of the leaf, 

 and also that the epidermal cells, unlike the interior 

 ones, have been emptied of their protoplasmic contents 

 and are rectangular in shape. It sometimes happens 

 that the epidermis consists of two or three layers instead 

 of one. 



The outgrowths of the epidermis, included under the 

 general term trichomas, have already been referred to ; 

 they must be regarded as part of the epidermal system. 



Fig. 226. — Cross-section of a leaf, showing epidermis above and below, 

 palisade cells under the upper epidermis, and loose tissue with intercellular 

 spaces below the palisade cells. (Gray-^ 



