Rosaceæ. 67 



The secondary wood often, but by no means always, 

 shows distinct annual rings. The growth-increment of the 

 first year is equally vigorous on all sides, but afterwards the 

 branch becomes highly hyponastic. The yearly growth on 

 the hyponastic side is naturally very different in different 

 localities; but it was always found to be less in the Arctic 

 and Subarctic material of D.octopetala, on an average 0.2 mm. 

 in younger branches, than in plants from the Botanic Garden 

 in Copenhagen where the average growth, likewise for younger 

 branches, was found to be 0.9 mm. Kihlman found an 

 average yearly growth of 0.07 mm. in the 108-years-old 

 branch mentioned above, and Schrøter measured a yearly 

 mean growth of 0.22 mm. in a 50-years-old stem from the 

 Alps, The maximum of the radial vessel-diameter was also 

 greatest in specimens from Copenhagen. — The chief deposi- 

 tory for reserve food-material in the two Dryas forms is 

 naturally the branches, and the tissues containing starch are, 

 in addition to the active part of the pith, the medullary rays, 

 which may be as much as 3 cell-layers thick, and several 

 cells in height, and here as in the root are connected by tan- 

 gential bands of wood-parenchyma. During winter the starch 

 disappears from the branches almost entirely and is replaced 

 by fat. In Copenhagen this change took place in November. 



In the peduncle there are no secondary formations. 

 Outside the numerous (about 20) primary vascular bundles 

 there is gradually formed a strong bast-cylinder which is 

 correlated with the fruit-bearing function of the peduncle; 

 there is no endodermis. The active cells in the pith have 

 weaker walls than have those in the stem. 



The leaf in D. octopetala and D. integrifolia is leathery and 

 especially in the former is highly reticulated upon the upper 

 surface. The lower surface is silver-white with a dense felt 

 of hairs; the upper surface is usually glabrous, but in several 

 forms it may be more or less hairy; for instance in Dryas 



5* 



