54 THE STRUCTURE OF ECONOMIC PLANTS 



Less frequently, they originate in phloem and ray parenchyma. 

 The organization of the root meristem may involve several layers 

 of procambial tissue in cases where the point of origin is not an 

 internodal region, or in proximity to a node which is still in the 

 meristematic condition and undergoing elongation. In older 

 stems, the adventitious roots frequently arise in the parenchyma- 

 tous ray tissue adjacent to the youngest secondary xylem and 

 phloem elements; and the root primordium may be regarded as 

 being derived from the ray or the interfascicular cambium. Some 

 roots may arise from the persistent pericycle under these conditions; 

 but the origin of roots from ray and secondary parenchyma derived 

 from the cambium is more common. 



Adventitious roots frequently occur at or near the nodal regions 

 where the tissues are usually meristematic, at least in young stems; 

 but the production of roots is not restricted to the nodal zone and 

 they may develop at any point along the internode. This is illus- 

 trated in the production of adventitious roots in the first internode 

 of Zea, and the formation of internodal roots in Lycopersicum. 

 (Fig. II.) A special case of adventitious root development occurs 

 in some crucifers in which the origin of the root is exogenous in- 

 stead of endogenous, involving epidermal and cortical tissues rather 

 than stelar ones. This was demonstrated by Hansen (4) for 

 Cardamine. In this case, the roots arise only in the axils of the 

 leaves and are related to the axillary bud, forming a lateral out- 

 growth from it. 



Adventitious roots may also arise from leaves, but this is less 

 common than their origin from stems. In the former, the root ini- 

 tials originate from parenchymatous tissues in close proximity to 

 the cambium of the vascular bundles. This occurs in the leaves of 

 Begonia and Bryophyllum. Crooks (3) demonstrated, in the 

 excised cotyledons of Linum, that adventitious roots originate 

 from a layer of parenchymatous cells on the adaxial side of the 

 veins. LaRue (7, 8) observed the rooting of excised cotyledons 

 of 41 plants belonging to 19 different families. 



The whole problem of vegetative reproduction, including the 

 production of adventitious roots, is associated with the retention 

 of the meristematic capacity on the part of certain cells of the plant 

 body. Where this occurs, the potentialities for further growth 

 and differentiation of structures are limited only by the environ- 

 mental conditions and the external and internal stimuli that may 



