68 



OUTLINES OF PLANT LIFE. 



behind the apical growing point. But 

 sometimes branches appear out of this 

 regular order. Such are called ad- 

 ventitious roots. (See^ 70.) 



Branches generally originate oppo- 

 site the wood strands, or with definite 

 relation to them. (See figs. 55, 56.) 

 The number of vertical ranks of bran- 

 ches can, therefore, be predicted with 

 some certainty from the structure of 

 the root, but the longitudinal intervals 

 at which the branches will be formed 

 cannot, because they are unequal (fig. 



55)- 



When secondary roots arise from 



the shoot, they have a fixed relation 

 to the leaves, or they are formed upon 

 the buds produced in the axils of the 

 leaves, or they may arise at indefinite 

 points along the internodes. In the 

 first case, roots may be produced 

 either opposite a leaf, or in pairs, right 

 and 1 eft of the base of the leaf. 



84. Origin. — The origin of root- 

 branches and of secondary roots is 

 rarely external ; that is, the root is 

 not commonly produced by growth at 

 the. surface of a member. In the great 

 majority of cases the origin of the 

 roots is internal ; that is, the forma- 

 tion of the root is begun by the growth 

 Fig 55.-Seediing pea, showing in the interior of the member pro- 

 lsX4tetataroot.'T^i;L'; ducing it. In most cases growth, 

 5£e°-AfSr1vank'' ^"'"^'"'^ begins Very near to the surface of the 



