74 



PRINCIPLES OF PLANT-TERATOLOGY. 



(1) On Roots. 



If the tap-root or radicle of a bean-seedling be cut 

 off at or near the tip, it is possible, as Groebel illustrates, 

 to induce very large numbers of lateral roots to grow 

 out which otherwise would never do so ; and, unless 

 one adheres unnecessarily closely to the ordinary 

 academic use of the term, these roots may be styled 

 adventitious. 



The tubers of Dioscorea are, it may well be assumed, 

 merely modified roots. Goebel figures an excised piece 

 of a tuber of D. sinuata which, on the anterior side, 

 produced numbers of adventitiuos roots, and on the 

 posterior side, shoots. 



(2) On Stems. 



These occur normally in Nature as a very widely- 

 spread phenomenon. They often, as in grasses, bulbous 

 Monocotyledons, and Selaginella, constitute, at least in 

 the mature state of the plant, the only existing root- 

 system. Originally roots, and roots only, formed the 

 seat of origin of other roots ; their formation by the 

 tissues of the stem must be a secondarily-acquired 

 character. When the normal root-system has been in 

 some way hindered from functioning properly, a com- 

 pensatory system of roots may arise from the stem- 

 tissues, as in the case of the vine and willow mentioned 

 by Masters. As a result of natural injury to the stem, 

 roots may arise near the injured portion, as in the 

 trunk of the elm cited by the same author, in which 

 the roots grew downwards in the hollow centre. The 

 writer observed in a stem of the Cycad Mncephalartos 

 AMensteinii, growing wild in Zululand, that, as a result 

 of external injury, great numbers of adventitious roots 

 had arisen in the cortex, some of them nearly an inch 

 in thickness, and were running down vertically through 

 the outer portion of that tissue. This is comparable 

 to the normal passage of adventitious roots in the 

 stem of some Marattiaceous ferns. 



