84 • Knud Jessen. 



wandering life. The shoot then takes more than two years 

 to develop. When a runner is entering on its last stage it 

 grows upwards toward the surface of the bog and at the 

 point where it bends the internodes usually become some- 

 what shorter, but by no means always, as easy transitions 

 from runners with 3 — 4 short internodes at the bending point 

 to runners which form aerial shoots without any shortening 

 of their internodes are met with. This difference expresses 

 the state of dependence in which plants growing in a bog 

 stand to the moss-vegetation of the latter. The shoots, 

 without the shortening of the internodes, were probably in 

 danger of being overgrown by the moss, consequently, no 

 short internodes have been formed but only elongated ones 

 to carry the terminal bud upwards to reach the light. 



The older plant probably develops only adventitious 

 roots; the latter arise from the nodes of the runners during 

 the second period of growth. 



Among related species this Riibus has no doubt an 

 unique shoot-structure which appears to make it specially 

 adapted for life in the habitats it has chosen for itself. 



Anatomy. The adventitiousroots are diarch— tetrarch. 

 The endodermis has rather thick walls which have become corky. 

 Even before the periderm, which is developed in the outermost 

 layer of the pericycle, is formed, consequently, in the absorb- 

 ent-root stage, the epidermis is destroyed and the two outer 

 layers of the cortex assume the appearance shown in Fig. 

 33, G. At a distance of a few mm. only from the root-apex 

 we find the epidermis dead, while the cortical layer next to 

 the outermost one only gradually attains the wall-thickness 

 figured, viz. about 2.5^«. The dead epidermis does not however 

 persist everywhere, as the figure indicates, often only frag- 

 ments of the radial walls are found. 



There are somewhat considerable differences in the ana- 

 tomical structure of the aerial and the under-ground stem 



