M/yUTh' STIircTl UK. 



fonniui,' ilie ui.i)fr surface, ami a similar layer fonniiiK 

 the lower surface. Tliese layers constitute the rj,i,l,riin'M 

 or skin of the leaf. Tlie outer part of the e|)itl«'rini8 is 

 usually a continuous layer, and is known as the rufolr. 

 It will be seen that the walls of these cells are nuu-h 

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

 also that the epidermal cells, unlike the interior one.s, 

 have been emptied of their protoplasmic contents and are 

 rectan}:;ular in shape. It sometimes liappens that the 

 epidermis consists of two or three layers instead of one. 



4'lie outgrow^ths of the epidermis, included under the general 

 terra trichomes, have already been referred to ; tliey must 

 be regarded as part of the epidermal system. 



An examination of the under surface of almost any leaf will 

 sliow the presence of a large number of oval openings, 

 somewhat similar to that shown in Fig. 277. These are 

 stomata (s, Fig. 27(;). They are formed by two epider- 

 mal crescent-shaped cells with a space between them, and 

 these have the power of separating or closing together 

 according to circumstances ; separating in the liglit, in 

 moist weather, and closing in dry. The openings com- 

 municate with inttirviillular spanus in the body of the 

 leaf, a number of which are seen in Fig. 27"). Fig. 278 

 is a larger view of a fulh* formed stoma (.v). In ordinary* 

 leaves with an upper and a lower surface, the stomata 

 are far more numerous on the lower side ; indeed, many 

 such leaves are entirely without stomata on the upper 

 surface. Vertical leaves have them rather e (ually dis- 

 tributed ou both surfaces. Immersed leaves and under- 

 ground stems have hardly any at all. and they are never 

 found on roots. 



The stems of Dicotyledons lose their epidermis at a compara- 

 tively' early period, and a tissue consisting of cells of cork, 

 filled witli air. takes its place. These cork-cells are modi- 

 fications of the cells beneatli the epidermis, and they form 

 an effectual protection to the tissues within. The skin of 

 the Potato-tuber exhibits this corky layer very clearly. 

 The special tissue from which the cork is developed is 

 called pheUogen. 



Fibro-Vascular System. • 



In the Fibro-vascular System different jihints exhibit a very 

 different arrangement of the component tissues. As a 

 rule, these tissues are capable of division into two group.s. 

 in one of which the wood is develoiied, and in the other 

 the bast. To the former of these groujis tlio general term 

 .ri/lem is applicable, and to the latter tlie term phlof.m. 

 The xylem is made up of the elongate<l woody cells with 

 pointed and overlapping ends, already referre<l to as fibrous 



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