5. DEVELOPMENT. 9 



the primary root and its branches. The leaf has, with rare excep- 

 tions (e.g. some Ferns), no persistent growing-point, and this is 

 also true of some stems. Members in which there is no persistent 

 growing-point are said to have limited growth. 



The growing-point adds, in the first instance, to the size of the 

 member to which it belongs, and is the means by which it grows 

 in length. But, in the case of the stem, it does more than this ; 

 it gives rise also to new members, either branches or leaves. It is, 

 in fact, the only source of origin of such new members. When 

 the growing-point is multicellular, the branches and leaves arise 

 from the superficial layers of cells ; so that their mode of origin is 

 exogenous. Moreover, these members are developed in a regular 

 order, such that the youngest of them are to be found nearest to 

 the growing-point, the older ones further away : this order of de- 

 velopment is termed acropetal succession. 



As a general rule, the secondary members are developed laterally 

 on the parent member, the latter continuing its growth in length. 

 But in a few cases, more particularly when the body is a thallus 

 or the stem is thalloid, the growing-point divides into two, each of 

 which forms the growing-point of a new secondary member simi- 

 lar to the parent. This mode of branching is known as dichotomy. 



In the root, the new members, except when dichotomy occurs, are 

 not developed at or from the growing-point, but at some distance 

 behind it. Unlike the branches and leaves produced by the stem, 

 the branches of the root are not developed at the surface, but from 

 a tissue lying deeply within the structure of the parent root. 

 (See Part II.) This layer, although it is situated among cells 

 which have become adult, retains its embryonic properties, and 

 gives rise to the growing-points of the several lateral roots. Hence 

 the origin of the secondary roots is endogenous, as their growing- 

 points are developed within the tissue of the parent root, and have 

 to force their way through it in order to reach the surface. The 

 order of development of the lateral roots is acropetal. 



It sometimes happens that secondary members are developed 

 out of their proper order or not in their proper place ; they are 

 then said to be adventitious. This is rare in the case of stems and 

 leaves, but is common in the case of roots. Roots are, for instance, 

 frequently developed adventitiously on stems, instead of from the 

 primary root which is their normal position. When they are de- 

 veloped on stems their origin is almost always endogenous. 



The secondary members commonly persist ; but frequently they 



