On the Structure of Roots. 485 



development, lying just behind the apex of the rootlet, where 

 the nascent cells are all alike. The central cord very soon dis- 

 plays traces of the structures called ducts, and the cells assume 

 the form, and more or less the substance, of the wood-cells 

 of the stem. Some important differences exist as to the arrange- 

 ment of their constituents in different classes of plants. In 

 Dicotyledons, such plants as turnips, beans, pease, our native 

 timber trees, &c., the structure of the central or woody part of the 

 root differs from that of the stem chiefly in the absence of a 

 central pith, together with the circumstance that the so-called 

 vascular structure consists of short-jointed ducts, without the more 

 flexible spiral vessels. 



In ordinary Dicotyledonous roots, when no tuberous develop- 

 ment occurs, the central woody structure soon acquires its dis- 

 tinctive character. The wood of the stem consists originally of 

 a number of perpendicularly arranged cords, standing in a circle 

 around the pith, a certain number of which pass out into each 

 leaf to form tlie skeleton of those organs. The lower portions, 

 inside the stem, extend down for a variable distance in different 

 plants. Those of the lower joints of the stem run down into the 

 roots to form its wood, so that here also we find the Avoody axis 

 at first in the form of distinct bundles, separated from each other 

 by cellular tissue {inedullari/ rays), but crowded closely together 

 in the centre, so that there is no pith. In the young root we find 

 the bundles belonging to the cotyledons largest, between these 

 the bundles belonging to a number of successive leaves. As the 

 stem has its leaves developed the number of these bundles is in- 

 creased, until at length a complete circle is formed (fig. 12). 

 When the stem has its joints elongated the _,. ,^ 



. . r iff. 12. 



number of bundles extending down into the 

 root is apparently more restricted than when 

 the root is crowned by a tuft of leaves. The 

 bundles belonging to the leaves, formed at a 

 certain height from the root, have their origin 

 at the points where some of the lower ones 

 run out into the leaves, so that they take the 

 place of the latter in the circle surrounding 



trie pith. ^ Cross slice of a young root 



When the root is not tuberous, the woody of groundsel, showing 

 bundles grow by the conversion of their cam- ^^'^ ^oody axis to be 



%•-,•• composed of wedge- 



biai tissue into wood and ducts, and soon shaped bundles separated 



form a solid mass of wood, the wedge-shaped by medullary rays. Mag- 



-.1.1 , T ... 1 1 1 nified 10 diameters. 



parts ol whicii are more or less distinguishable 

 in different cases. Sometimes the medullary rays separating 

 them remain tolerably large ; in other cases these are lost sight 

 of, and the separate bundles are then often only roughly trace- 

 able by the arrangement of their larger ducts. 



