Rosaceæ. 115 



summer previous to the year in which they expand, viz. 

 Alchimilla alpina, Alch. vulgaris, Dryas octopetala, Potentilla 

 emarginata and Riibus saxatilis, all of which flower in June. 

 It is probable that the great majority of the species behave 

 as these, with the exception however of Pot. palustris in 

 which the young flower-buds have not been observed 

 until the spring of the year in which they expanded. 



C. The absorbent roots often present anatomical peculi- 

 arities, for instance, a thin epidermis which is usually 

 more or less collapsed is found in the majority of the 

 Potentillas {anserina, emarginata, maciilata, nivea and 

 pulchella), and in addition in Sibbaldia and Rubiis saxatilis. 

 In Rubus chamæmoriis it is destroyed early. A stronger 

 epidermis occurs in Pot. palustris, where it is very small- 

 celled, and in the Alchimilla-sipecie& where there is a ten- 

 dency to throw it off entirely, because the thin-walled 

 exodermis ruptures. In Pot. emarginata, P. nivea and P. 

 pulchella the radial and the outer walls of the exodermis 

 are very highly thickened, and this cell-layer is also very 

 strong in Rubus chamæmorus. In the Alchimilla-sipeGies 

 the cortical layer within the thin exodermis is fairly 

 strong. In Dryas, Rubus arcticus and R. saxatilis the 

 radial walls — and in the two last species also the outer 

 walls — of the cortical layer outside the endodermis are 

 very strong. 



Ectotrophic mycorrhiza occurs in Dryas; and endo- 

 trophic mycorrhiza has been found in Pot. anserina. 

 Pot. emarginata, Pot. nivea, Pot. palustris, Pot. pulchella, 

 Sibbaldia, Alchimilla alpina, Alch. acutidens and Rubus 

 arcticus. All the Rosaceæ mentioned here have probably 

 mycorrhiza, except Rub. chamæmorus. 



D. A continuous cambium is found in the perennial portion 

 of the shoots in all the species. According to the nature 

 of the tissue produced by the cambium the species may 



8* 



