3.] 



CHAPTER I. GENERAL MORPHOLOGY. 



19 



or it may sooner or later pass over into the adult condition, when 

 the growth of the member is said to be limited. Leaves usually 

 have limited growth ; occasionally roots (e.g. the primary roots of 

 Monocotyledons), and also shoots (e.g. dwarf-shoots). 



c. The Development of the Secondary Members. After the differ- 

 entiation of the primary members of the embryo, all other members 

 are developed from them and are generally termed secondary. 



When the segmentation of the body is complex, various orders of members 

 may be distinguished. Members borne directly on the primary members are 

 said to be secondary members of the first order ; those borne on secondary 

 members of the first order are said to be of the second order, and so on. 



Two kinds of secondary members may be distinguished: the 

 normal and the adventitious. The distinction depends upon (a) 

 the place of origin, and (b) the order of development of the mem- 

 bers. For instance, when a stem or a leaf bears a root, that root 

 is adventitious, because secondary roots are typically only pro- 



FIG. 8. Dichotomous branching of Dictyota dichotoma (after Naegeli). A, growing- 

 point with apical cell before division; B, growing-point with divided apical cell; C, de- 

 velopment of the dichotomous branches. 



duced by primary roots ; again, when a member is developed out 

 of its proper order, that member is adventitious. 



Secondary members may or may not be developed from the 

 growing-point of the parent member. 



The normal secondary members are developed from the growing- 

 point of the parent member. 



The growing-point may produce secondary members either by 

 dividing into two (dichotomy) or by lateral outgrowth. 



The result of dichotomy is, in all cases, to give rise to new mem- 

 bers, which are similar to the parent member and to each other. 

 In no case are morphologically dissimilar members produced by 

 dichotomy. Dichotomy is therefore a form of branching (see p. 5). 



The product of lateral development may be either a similar or a 

 dissimilar member ; if the former, it is a case of lateral branching. 



The normal secondary lateral members of the shoot (i.e., leaves and 

 branches) make their appearance, in most cases, as outgrowths 



