Root Structure 



31 



In Dicotyledons there is, as a rule, a small number of such 

 bundles, whilst in the roots of Monocotyledons the number is 

 considerably larger. 



Surrounding the bundles there is a ring of specially modi- 

 fied meristem cells known as the pericambium, and outside 

 this an enveloping layer of cells forming the bundle-sheath or 

 ertdodermis. Branches of the root are formed by outgrowths 

 of the pericambium which push their way through the adjacent 

 tissue, and thus form rootlets, which repeat the structure of the 

 original root (fig. 51). 



Roots increase in thickness by the formation of a ring of 



Fig. 51. — Longitudinal section of main root of 

 Victa. Faba. ?-, r, cortex of main root : /, 

 fibro-vascular bundles ; «, «, lateral rootlets 

 developing from l^ericambium, and breal:ing 

 through cortical tissue ; h, pileorhiza of side 

 rootlets. (After Prantl.) 



Fig. 52. —Epidermis of root with 

 hairs. 



meristem between the xylem and phloem bundles, and which, 

 by the repeated division of its cells, produces new xylem and 

 phloem. 



The cells of the epidermis often possess prolongations or 

 hairs {trichomes), which tend to increase the amount of surface 

 in contact with the soil (fig. 52). 



FORMS OF ROOTS. 



There are certain terms which are used in the description 

 of the form of the root. When it is broad at its base and 



