3.] CHAPTER I. GENERAL MORPHOLOGY. 21 



The point at which a leaf is developed on the stem and remains 

 attached is termed the insertion of the leaf ; and the insertion of a 

 leaf, or those of two or more if at the same level, marks a node of 

 the stem, and the portion of the stera^ whether long or short, be- 

 tween two consecutive nodes is the internode. When two or more 

 leaves are inserted at each node, or when the single leaf has a 

 broad sheathing insertion- (e.g. Grasses), the nodes of the stem are 

 strongly marked, giving it a jointed appearance. 



The normal secondary lateral members of the root are not developed 

 directly from the growing-point of the parent root, but at some 

 distance behind it where the tissues are already differentiated. 

 They are developed endogenously ; that is to say, the growing-point 

 is formed by the division of one or more cells belonging to an in- 

 ternal layer of tissue of the parent root, so that the young root 

 has to penetrate the cortical tissue before reaching the surface.. 



Adventitious members are of frequent occurrence, and are 

 generally not developed from the growing-point of the parent 

 member, but their growing-points are new formations. 



Adventitious branches may be developed on the margin or sur- 

 face of a thallus (e.g. Metzgei'ia, Pellia, etc.). 



Adventitious shoots are commonly developed from the callus (see 

 page 214) which covers the cut surfaces of wounded parts (e.g. 

 development of shoots from pieces of leaves of Begonia, Achimenes, 

 etc.) ; or they may be developed, independently of any injury, 

 on a leaf (e.g. Aspidium Filix Mas, Bryophyllum calycinum, 

 Gardamine pratensis, etc.) ; or on a root, as in a large number of 

 trees and other plants : the development of adventitious branches 

 on a stem is comparatively rare {e.g. Psilotum, Begonia; some 

 Liverworts, such as Jungermannia bicuspidata, etc.). 



Adventitious shoots are, in most cases, developed exogenously ; 

 but endogenous development has been observed (e.g. in some 

 Liverworts, in roots of Anemone sylvestris, Ailanthus glandulosa, 

 Sium latifolium). In some cases they are formed by the direct 

 conversion of the growing-point of a root (e.g. Neottia Nidus-Avis, 

 Oatasetum tridentatum, see p. 7). 



The true adventitious shoots should be clearly distinguished from those 

 apparently adventitious shoots which are due to the overgrowth of a normal 

 but dormant bud (see p. 32) by the surroundiug tissues (e.g. Equisetum, 

 Gleditschia sinensis and triacanthus, Symphoricarpus vulgaris, etc.). 



Adventitious roots may be developed from a callus, or from.leaves 



