484 



On the Structure of Roots. 



it becomes very large ; when it undergoes decomposition in pro- 

 portion as it is renewed behind, it presents an irregular ragged 

 pj„ Y\ appearance, which probably 



gave rise to the idea of a 

 spongy structure at the end of 

 the rootlets. 



In some roots the epider- 

 mis produces no Jlbrils, but 

 remains smooth. Tliis is 

 especially the case in the 

 delicate filamentous roots, 

 annually thrown off, of many 

 Monocotyledons, as of the 

 onion, hyacinth, crocus, &c. 

 In these roots the epidermal 

 cells retain their general de- 

 licate character throughout 

 their existence ; and probably 

 the roots of this character 

 absorb by their surface 

 throughout their whole length ; 

 while in woody roots the ab- 

 sorbent action is confined to 

 the rootlets — to the regions 

 near the growing points, — 

 where the epidermis is still 

 delicate and covered with its 

 hair-like fibrils. 



In woody roots, as the whole 

 organ increases in size and 

 the internal part becomes lig- 

 nified, the cortical region 

 changes its character. The 

 epidermis dries up, and its 

 place is taken by a corky 

 structure, formed of two or 

 three layers of the cells previously subjacent to the epidermis. 

 When this change has taken place the direct absorbent power 

 may be regarded as lost Simultaneously with this change the 

 inner cortical parenchyma often increases considerably in quantity, 

 and this is particularly the case in fleshy roots, where this region 

 subsequently becomes tlie reservoir of accumulated nourishment. 

 The centre of a very young root is occupied by a cord of 

 cellular tissue of different form from the cortical parenchyma, 

 consisting of elongated cells, the cambium of the future wood, 

 which merges near the growing point in the focus of cell- 



rerpenrticular slice of the end of a rootlet of barley, 

 showing the dissolving cellular tissue at the point 

 which lias been mistaken for a spongy structure. 

 Magnified 50 diameters. 



