132 



GENERAL BIOLOGY 



Supporting tissues, necessitated 

 weight. 



Conducting 



by increasing size and 



tissues, necessitated by 

 the increased distance between the two 

 sources of intake of food materials, the 

 rhizoids deep in the soil, and the assimi- 

 latory parenchyma of the leaf, lifted 

 high in the air. 



The position of these new parts in 

 the plant body can best be learned by 

 an examination of the structure of the 

 mature stem in a cross section of 

 which (fig. 82) they may be seen with 

 the unaided eye. As before, the sur- 

 face layer of cells is epidermis, and the 

 whole of the soft part of the interior is 

 parenchyma. The new tissues whose 

 function is chiefly or wholly supportive 

 are more or less brownish in color and 

 arranged i) in a peripheral layer just 

 beneath the epidermis, and 2) in two or 

 more broad, darker colored strands of 

 tissue extending through the midst of 

 the parenchyma. These tissues consist 

 of thickened and closely united walls of 

 empty cells. The inner darker strands 

 may readily be traced through the soft 

 parenchyma, and followed where they branch out into the 

 bases of roots and leaves. 



The tissues whose most important function is the conduc- 

 tion of food materials (though certain of their elements 

 also serve for support) are found in the vascular bundles, 

 which, also are readily seen in cross section of the stem. 

 They are of various sizes, and not constant in number, but 



FIG. 81. Leaf epidermis 

 of the fern ; st, stomates 

 or leaf pores. 



