STRUCTURE OF RADIAL BUNDLES. 



355 



Papilionaceae (Pisum, Phaseolus). The fibrous bundle lies inside the pericambium. In 

 other respects the phloem still requires more exact histological investigation in these 

 cases. 



In all roots of Dicotyledons investigated a pericambium, consisting of one, or in 

 many cases of several layers, completely surrounds the xylem-pktes. Those peculi- 

 arities of its structure which are related to the development of lateral roots, the resin- 

 canals which sometimes occur in it, and other points connected with it, will have to 

 be discussed below (Sects. 117 and 133). 



This original structure of the roots of Dicotyledons is, however, permanent in but 

 few cases ; in most cases, and in 



many species immediately after its , ~\'^^r^^^<>^^J \^ 



origination, it is altered by the 'C^^^'Cy^^^^^amQCy' 



secondary growth in thickness, '^ ^^ 



starting from the inside of the 

 phloem-rays, of which we shall 

 treat in Chap. XIV ; comp. Fig. 

 165. Hence result essential and 

 actual differences from other roots, 

 especially those of Ferns and Mono- 

 cotyledons, in which, with the ex- 

 ception of many roots of Dracaena *, 

 these secondary changes are want- 

 ing. It must, however, be expressly 

 stated that the changes due to 

 secondary growth in thickness occur 

 in by no means all roots of Dicoty- 

 ledons, and thus do not establish 

 any generally valid distinction be- 

 tween these and the others. Apart 

 from those cases where, as in the 

 subsidiary roots of Stachys silvatica, Mentha aquatica, Lysimachia nummularia, Myrio- 

 phyllum, and Hippuris, the secondary growth in thickness is infinitesimally small, and 

 as such even doubtful, because in other cases also the innermost vessels uniting the 

 plates are developed very late, this secondary growlh is completely absent in a number 

 of subsidiary roots. For instance itjlHdse of Gunnera ^ the Nymphseaceae, Ficaria 

 ranunculoides, and Primula Auricula, to which cases it may be anticipated that more 

 extended investigation will. a^cJ-Qthers. The fact that a rudimentary secondary growth 

 in thickness occurs at the points' of insertion of the roots in questibn (in Ficaria and 

 Nuphar ') has no effect on the condition of their much greater portion. 



The fact that in roots of Dicotyledons sclerosis of the endodermis but rarely 

 occurs no doubt stands in the closest relation with the occurrence of secondary 

 growth. Such sclerosis, however, occurs for example in the adventitious roots on the 

 rhizome of Prirriula Auricula and .Ranunculus repens ; comp. Figs. 164 and 165. 



->^o^o;'A°g^o 



Fig. i64.~-Primula Auricula (225). Cross-section through the heptarch 

 vascular bundle of an adventitious root and its surrounding^ tissue, ^peri- 

 cambium ; £ the external primordial vessels of the xylem rays, which 

 alternate with the same number of phloem groups s, and are separated 

 from the latter by thin-walled parenchyma; w endodermis, outside which 

 is tolerably thick-walled cortical parenchyma, with intercellular spaces 

 quadrangular in cross-section. 



^ Compare Caspary, Pringsheim's Jahrb. 

 Reinke, Morpholog. Abhandl. p. 58. 



A a 2 



I. p. 446. — Falkenberg, /. c. p. 197. 



^ Van Tieghem, /. c, p. 266, &c. 



