THE STRUCTURE OF THE ROOT 15 



that the groups of wood vessels (Fig. 4 g) have ■ become en- 

 larged by the addition of new vessels and wood cells, so as to 

 touch each other laterally, and then we should have a con- 

 tinuous cylinder of wood such as we find in any first year's 

 twig. 



Between the dark groups of wood vessels we see in Fig. 4 s 

 lighter groups of smaller cells. These are the more delicate 

 passages of the soft bast (leptom), which, we have previously 

 mentioned, conduct through their sieve-like transverse walls 



'-CD a 



'-^^^^O^Qr 



Fia. 4.— Sbvbn-Eayed VAsonLAR Cylindek op an Adtentitiocs Root op 



Primuta Auricula. 



g, wood rays ; «, groups of soft tast cells ; p, pericambium ; «, bundle-sheath (after De B art). 



the plastic food substances formed in the leaves to the root 

 system, for the furtherapce. of its growth. The tissues for lie 

 upward passage of the raw mineral salts which are absorbed 

 by the roots lie at the side of the tubes, which have to con- 

 duct the formed substances down from the leaves. Both tissues 

 ■are .surrounded by a ring of lightly coloured cells,, p, the 

 pericambium. It is in this layer that . the first rudiments of 

 the new lateral roots arise, and in such places the surround- 

 ing ring of thick- walled cells which protects all the conducting 

 tissues is interrupted. ; This ring (Fig., 3 s, and Fig. 4 u) is 



