422 



^villl plants which overshatlow it (Norman). In Nova Zembia il 

 grows npon dry slopes (Ekstam). 



Geographical Distribution. Arctic America, East 

 and West Greenland, Iceland, the Færoes, Scandinavia, Finland, 

 the Alps, Caucasus, Arctic Russia, Nova Zembia, North Siberia, 

 Altai and Asiatic coast of Bering Strait. 



Anatomy. The adventitious roots of the first order. 

 The primary cortex is thrown olT or else persists as dead layers 

 around the secondary formation. In a young root from Iceland, 

 gathered in May, tangential lacunae had been developed between 



the endodermis and the primary 

 cortex, the outer layer of which was 

 somewhat collenchymatously thick- 

 ened, while the epidermis was thin- 

 walled and corky. The secondary 

 cortex resembled the primary in the 

 fact of its outer layers being also 

 somewhat coUenchymatous; the inner 

 layers were thin-walled and distinctly 

 radially arranged. The thin-walled 

 endodermis had replaced the primary 

 epidermis; its cells were tangentially elongated. The central 

 cylinder is triarch or tetrach or according to Marié, even 

 pentarch or hexarch: the secondary vessels were larger than 

 the few primary ones. Tlie cambium was complete. 



The roots of tlie second order remain in the primary 

 condition and are diarch. The endodermis is slightly thickened. 

 The cortex consists of four thin-walled layers, of which the 

 innermost is the largest. The epidermis is remarkable owing 

 to the fact that it is dimorphic (Fig 54); it is partly of thin- 

 walled cells which collapse at an early period, and partly of 

 cells of which the walls thicken at an early period (Freidenfei.t). 

 These roots contain spongy hyphae. 



The rhizome, also, has secondary growth. The youngest 



Fig. 54. Thalictntm alpimim. 



Fragment of transverse section of 

 « root of second order (Iceland: Hav- 

 nefjord; *^li). ep , Epidermis; hrj, 

 hyphae; end, endodermis. 



