OEGAKS OF NUTRITION. 



117 



The true or primary root, from its being formed by a direct 

 elongation of the radicle, generally continues to grow downwards 

 for some time at least, and hence forms a main trunk or axis 

 from which the branches are given o^{fig. 227). Such a root 

 is termed a tap-root, and may be commonly observed in Dicoty- 

 ledonous Plants. On the contrary, the roots of Monocotyledonous 

 and Acotyledonous Plants, which are adventitious, are usually 

 of nearly equal size, and given off in variable numbers from 

 the radicle {fig. 225, r). Some adventitioxis roots, such as those 

 called aerial, require a more particular notice. 



Aerial Roots. — The simplest forms of such roots are seen in the 

 Ivy {fig. 195) and some other climbing plants. In these plants 

 they are simply intended for mechanical support, and not to ob- 

 tain nourishment for them : this they obtain by their ordinary 



Fig. 228. 



s^Fn,-.CN:s 



mmm 



Fig. 228. The Banyan Tree {Ficus indica). 



roots fixed in the soil. From some recent experiments how- 

 ever, on the Ivy, it is probable that some nutriment at least 

 may be taken up by these roots. In other plants, however, the 

 aerial roots which are given off by the stems or branches descend 

 to the ground, and fixing themselves there, not only act as 

 mechanical supports, but also assist the true root in obtaining 

 nourishment. Such roots are well seen in the Screw-pine {Pan- 

 daniis odoratissimus) {fig. 170, 2.) in the Banyan or Indian 

 Fig-tree [Ficus indica) {fig. 228), and in the Mangrove tree 

 {Rhizophora Mangle) {fig. 229). In the latter tree these aerial 



