190 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 34 



or even the secondary bast. In both these cases the adventitious 

 root is developed endogenously. 



In other cases, adventitious roots are developed from more 

 superficial layers of tissue (e.g. Equisetum, Selaginella; roots 

 developed in connection with buds, as in Ranunculus Ficaria, 

 Nasturtium, Cardamine, Anemone, etc.) that is, exogeiiously. 



The adventitious development of buds on the stem may take place 

 either exogeiiously or endogenously. In the former case the buds 

 may be developed each from a single epidermal cell (e.g. Begonia 

 prolifera, underground shoots of Psilotum), or from the epidermis 

 and subjacent layers (e.g. Linaria vulgaris}. In the latter case 

 the adventitious bud arises from the pericycle (e.g. Cuscuta, 

 epicotyl of Convolvulus arvensis). 



Adventitious buds are commonly developed in the Algaa and Liverworts, 

 either exogenously, from single superficial cells at the margin or on the surface 

 of tbe thallus of Delesseria and other Alga?, or of that of Metzgeria furcata 

 among Liverworts, or endogenously, as in Fucus and apparently also in 

 Metzgeria. 



2. On the root. Adventitious buds may be formed either exo- 

 genously or endogenously on the root ; in the former case they 

 arise from the superficial layers (e.g. Aristolochia cleinatitis) ; in 

 the latter, from the pericycle (e.g. Alii aria offi,cinalis. Anemone 

 sylvestris, etc.) 



3. On the leaf. Adventitious buds developed on leaves are of 

 exogenous origin, the epidermis being more especially concerned 

 in their production. Adventitious roots are usually of endogenous 

 origin, being derived from cells of the pericycle ; but in some 

 cases (e.g. Cardamine pratensis, Nasturtium officinale and silvestris}, 

 where they arise in direct connexion with adventitious buds, 

 they are exogenous, being derived from the epidermis and the 

 superficial cortical layers of the axis of the bud. 



Adventitious buds and roots are also developed from the callus (see p. 21) 

 formed on the injured surfaces of stems, roots, and leaf-stalks : the former may 

 be endogenous or exogenous, the latter are endogenous. 



From the foregoing account of the development of adventitious 

 buds and roots, it is clear that no definite rule as to their mode of 

 origin can be laid down, based on their morphological character. 

 However, the following rule appears to hold good, that the mode 

 of origin of an adventitious member depends upon the age of the 

 part from which it is developed when the part is very young, the 



