INTERNAL ANATOMY OF ORGANS. LEAVES. 



539 



into a row of fibres radially arranged and imbedded in a great quantity 

 of lax parenchyma. The libro-vascular bundles converge at the " collar," 

 and then separate again to surround the pith of the stem ; they also con- 

 verge again towards the point of the root. These so-called roots are 

 more nearly allied to the stem, and are, indeed, hypocotyledonary stems. 



The structure described under the name of spongiotes has no existence 

 in nature. The error has probably arisen from the appearance presented 

 by the pileorhiza. 



Old roots of Dicotyledons present a dense heart- wood like the trunks, 

 passage of fluid taking place through the outer layers. When the older 

 parts of roots are exposed to the air by removal of soil, they acquire a 

 thick corky periderm. 



The general structure of the root of Gymnosperms is like that of Dico- 

 tyledons, except that there is no dermatogen, the outer layer of the 

 p'eriblem becoming transformed into epidermis. 



Fig. 585. 



Projection of a fragment of the leaf of the Turnip, constructed from sections made in various 

 directions, and magn. 100 diam. : e, epidermis of the upper surface with its stomata ; e , 

 epidermis of the lower face ; , stomata, cut through, opening into intercellular cavities ; 

 TO, close parenchyma (palisade cells) of the upper part of the leaf; m', loose and spongy 

 parenchyma of the lower part ; f.b, the cut end of a fibro-vascular bundle forming on 

 of the veins of the leaf. 



Structure of Leaves &c. The plan of construction of the leaves 

 and of the other appendicular organs of the stem is in the main 

 identical throughout all cases; but there is very considerable 

 variation within the limits of the general type. The essential 

 character of the anatomy of a leaf is, that it is an expanded layer 

 of parenchyma clothed over its whole surface with epidermis, and 

 furnished, according to its degree of development, with a more 



