SOLANUM TUBEROSUM 



52-3 



definite root cap has been organized and the stelar elements have 

 begun to differentiate against the primary xylem and phloem of 

 the main axis. Direct contact with the primary vascular elements 

 of the stele is possible owing to the point of origin of the lateral 

 root and its early initiation. 



Origin of Adventitious Roots. — In commercial practice, the 

 propagation of the potato, except for experimental work in potato- 

 breeding, is accomplished by vegetative methods in which por- 

 tions of the tubers, known as "seed pieces" or "seed", are planted. 

 Vegetative growth results 

 in the development of 

 shoots, adventitious roots, 

 and tubers. In this case, 

 the entire root system is 

 adventitious; and, as pointed 

 out by Priestley and 

 Swingle (17), the roots do 

 not arise directly from the 

 tuber, but originate from 

 young sprouts which are 

 formed by the development 

 of the buds or "eyes" of the 

 seed piece. These roots fre- 

 quently occur in groups of 

 three, and their point of 

 origin is in the pericyclic 

 region of the subterranean 

 portion of the stele in close proximity to a nodal plate. As 

 in the origin of laterals, the first evidence of adventitious root 

 formation consists of an activation of the pericyclic cells, which 

 first elongate radially and then divide tangentially. The endo- 

 dermis keeps pace with the formation of the root primordium for 

 a time, and later is broken when the endodermal cells cease divi- 

 sion. Artschwager (i) states that the developing rootlet "pushes 

 its way mechanically through the cortex, and is aided by the 

 dissolving action of enzyms which are probably secreted by the 

 cells of the endodermis." 



The ontogeny of the lateral and adventitious roots is similar 

 to that of the primary root except that they are not always diarch. 

 Lateral roots are usually diarch, but adventitious roots may fre- 



'- — 'pel 



Fig. 177. Transection of adventitious root : ca, 

 cambium ; co, cortex ; en, endodermis ; e^, epidermis ; 

 mx, metaxylem ; ^cl, pericycle ; ^h, phloem ; fx, 

 protoxylem ; xy 1, secondary xylem. 



