94 Knud Jessen. 



walls mentioned above, and the epidermis turned yellow 

 when treated with chloride-zinc-iodine. (Fig. 36, E). 



The anatomical structure of the annual aerial shoot is 

 naturally very different from that of the perennial rhizome. 

 In the latter secondary wood is developed which forms annual 

 rings, and the primary cortex is thrown off, a periderm of 

 about 6 layers without intercellular spaces and consisting 

 chiefly of cork being formed in the pericycle. The latter, 

 a few layers of phelloid-cells, the medullary rays, which are 

 of the same structure as those of the root, and especially the 

 peripheral part of the pith are depositories for starch. The 

 aerial shoot remains in a far higher degree in the primary 

 condition. A continuous cambium in the lower part forms 

 only a small quantity of wood, and it is likewise only in the 

 lower portion that a periderm is found like that shown in 

 Fig. 36, F. In the cortical layer bordering on the bast-ring 

 tangential walls are developed, the walls in the outer layer 

 thereby formed become corky, but the inner layer which 

 corresponds to the phellogen remains unaltered. This rudi- 

 mentary periderm distinguishes with certainty the stem of Rub. 

 arcticus from that of Rub. Chamæmorus. — The bast, which is 

 absent from the rhizome, is about 3 layers thick in the lower 

 part of the stem and forms a continuous ring, becoming 

 weaker upwards. Endodermis is absent, but is present in the 

 perennial part of the stem. The outer wall of the epidermis 

 is rather thick and has a well-developed cuticle. Within 

 the epidermis there are two layers of slightly developed 

 collenchyma. — The pith has the usual peripheral layer of 

 starch-containing cells, but in the central part I have seen 

 no such cells. Trecul (1. c.) who has described the pith 

 records that in the central part a few isolated cells were found 

 containing tannin. 



The leaf, which probably as a rule lives one summer 

 only (see p. 93), has the usual mesophyllous structure; 



