368 



suberized. A somewhat thickened, subexodermal layer occurs 

 and the cortex has larger or smaller lacunae. The endodermis is 

 somewhat thickened and the central cylinder is diarch to tetrarch. 



Fig. 20. R. sulphureus. 

 A, Portion of section of central cylinder of adventitious root of the first order (Scoresby. 

 Sound; i''Vi). Б. Transverse section of rhizome (Spitzbergen ; ^ii). C, Portion of trans- 

 verse .section of stem (Spitzbergen; about '-"/i) ; b, bast; c, cortex; end, endodermis; g 

 the large pentagonal sieve-tube; I, lacuna; m, pith; mh, pith-cavity; p. pericycle; ph, 

 leptome; vp, lignified parenchyma; x, xylem. 



There is a large sieve-lube on the outside of each group; 



cambium is absent (Fig. 20, A). 



The root of the second order consists of epidermis, 



about five layers of cortical ceils and a diarch central cylinder. 



The epidermis as well as large 

 parts of the cortex are col- 

 lapsed; a thickened subex- 

 odermal layer does not occur. 

 The epidermis and the in- 

 distinct exodermis are suber- 

 ized. Here also a large sieve- 

 tube occurs on the outside 

 of each group of sieve-tissue. 

 Mycorrhizas have been 

 found in specimens from all 

 the habitats and the hyphae 

 often occur in large quantities 



in the lower, slender parts of the roots of the first order and 



in the roots of the second order. Balls of hyphæ, whenever 



such occur, are most frequently found in the inner cells of 



the cortex (Fig. 21). 



Fig. 21. R. sulphureus. 



Portion of transverse section of root of 



the second order showing mycorrhizas (Nova 



Zembla; 220/1); ep, epidermis; ex, exodermis; 



end, endodermis; Л, hyphæ. 



