346 TRANSACTIONS AND PROCEEDINGS OF Til K [ Sess. LXiv. 



comparatively early period, and apparently with remark- 

 able regularity, the initial stages being found only during 

 tlie formation of the first ring of wood, and in general 

 shortly before or shortly after it has reached half its 

 ultimate thickness, though here considerable variation may 

 . occur. 



The greater preponderance of root-bearing papillre, which 

 may frequently be observed on old than on young stems, 

 is probably to be explained by the greater proximity 

 of the former to the surface of the soil, and the con- 

 sequently greater degree of moisture in the surrounding 

 atmosphere during the period of their development. 

 However this may -be, the roots underlying these papillae 

 may, in the great majority of cases, be traced back without 

 difficulty to within the first ring of wood, even when 

 the stems on which they appear are eight or nine years of 

 age, and one or two centimetres in diameter. 



The young roots generally bear a definite relation to the 

 primary vascular bundles of the stem, which, as has been 

 already pointed out, are of considerable tangential width, a 

 point wliich has here a certain importance, as the roots 

 do not usually arise opposite the median plane, but generally 

 nearer either the riglit or left hand edge of a bundle, 

 so that papillae are seldom placqd directly on the longi- 

 tudinal ridges, which in the young stem are the superficial 

 indications of the subjacent bundles, but in most cases on 

 the intervening flat surface. 



Though the situation of the root on the circumference of 

 the stem is thus to a certain extent defined, no rule whatever 

 appears to be ibllowed with regard to its longitiidinal position, 

 and papilhe seem to arise with equal readiness on the bundle 

 throughout any part of its course. 



Van Tieghem and Douliot (8), in their account of the 

 origin of lateral rootlets, state that the adventitious roots 

 which arise from the underground stems of S. hiherosum 

 originate in divisions taking place in the single layered 

 pericycle, while tlie cells of the phloem parenchyma assist 

 in the formation of the basal part of the central cylinder; 

 and the endodermis forms a digestive cap. 



In the species now under investigation, tin; roots, on the 

 <jther hand, apparently <nvc their origin Lo the proliferation 



