18 



MANUAL OF BOTANY 



Fig. 19. 



Fig. 20. 



is always internal, and protected by a thin cushion or cap of 

 tissue known as the root-cap (h, fig. 18; r,fig. 19). In those 

 roots which grow in ordinary soil, the root is furnished, a little 

 way behind the apex, with a number of very delicate outgrowths 

 Erom the surface, which penetrate into the crevices between the 

 particles of the soil, and come into very close relationship with 

 them. These structures are known as the root-hairs. They 

 are not distributed over the whole surface, but only occupy a 

 small space not far behind the growing part (fig. 20). These 

 hairs are not to be regarded as appendages, in the sense men- 

 tioned above, for all parts of the plant agree in being able to 



give rise to similar 

 structures. They 

 are only outgrowths 

 of the external 

 covering. 



The direct pro- 

 longation of the 

 radicle downwards 

 forms what is known 

 as the primary root. 

 This may grow to a 

 great size, persisting 

 through the life of 

 the plant. It is 

 usually tapering in 

 form, and bears 

 numbers of bran- 

 ches, which them- 

 selves may branch 

 repeatedly and so 

 give rise to a large 

 root- system. When the primary root is thus persistent and 

 stronger than its branches, it forms what is known as a tap- 

 root. On the other hand, it is not unusual for the primary root 

 to develop only to a slight extent, and to be speedily surpassed 

 in vigour by its branches. 



The branches of the primary root, like the latter, always 

 have their apices covered by a root-cap. They arise, with very 

 few exceptions, within the tissues of the primary root, and in 

 their growth bore their way outwards through its external 

 layers. They are known as secondary roots, and they in turn 

 produce tertiary roots, and so on. In their order of develop- 



Fig. 19. Young plant of Lemna {Duckteeed). r. Roofc- 

 cap. 7^117. 2U. Xlltimate branches ofa root, show- 

 ing position of root-l.airs. 



