116 Knud Jessen. 



be divided into two main groups. One, more herbaceous, 

 in which the secondary formations consist of groups of 

 vessels rich in parenchyma and separated by broad medul- 

 lary rays; the Potentillas with the exception of Pot. 

 palustris and Pot. tridentata belong to this group. Another, 

 more lignified, in which the cambium produces a ring- 

 shaped woody portion; to this belong the rest of the species. 

 In Dr y as, Potentilla palustris., P. tridentata, Sibbaldia and 

 Rubus are found medullary rays, one to a few cells broad; 

 medullary rays are absent from Alchimilla except above 

 the leaf-trace bundles ("Blattspurstränge"). In Alch. al- 

 pina and especially in Alch. vulgaris and Alch. færøensis the 

 secondary wood-formations constitute only a compara- 

 tively small portion of the shoots, and these are then more 

 herbaceous. 



Bast occurs regularly only in Drijas. 



In Dryas, from which, as already mentioned, an 

 endodermis is absent, cork is formed in the inner part 

 of the cortex; in all the others which have an endo- 

 dermis, a periderm is probably always formed in the 

 outermost layer of the pericycle (cf. Solereder^). Dryas, 

 only, has a scaly bark ("Schuppenborke") formed of cen- 

 tripetally developed phellogens which develop only cork- 

 cells without intercellular spaces. In the other species, 

 on the other hand, the periderm consists of one layer of 

 cork-cells alternating with usually three layers of phelloid- 

 cells. It contains large intercellular spaces, which are 

 however absent from the inner side of the cork-layers in 

 the great majority of the species. 



SoLEREDER (1. c.) who also describcs the periderm has 



found intercellular spaces in the phelloid in Pot. palustris 



and Poterium sanguisorba; in his opinion this suggests a 



resemblance to aërenchyma and he regards it as an adap- 



» Anatomie d. Dicotyled. Stuttgart, 1899, p. 348. 



