118 Knud Jessen. 



phelloid, the pericycle, the medullary rays and the wood- 

 parenchyma; and in the pith, especially the peri- 

 pheral part. 



The reserve food-material consisted especially of 

 starch in all the species which have been investigated in 

 regard to this point ; but in several of these species (Dryas, 

 Alchimilla, Pot. emarginata, Pot. nivea, Pot. palustris, Sib- 

 baldia, Rubus saxalilis) it has been demonstrated that 

 during winter the starch disappears more or less com- 

 pletely and is replaced by fat. 1 Probably during autumn 

 the starch is converted into fat, and then during spring 

 the fat is again converted into starch. Similar conditions 

 are well-known as regards several trees. It is apparently 

 a common occurrence among perennial herbs and dwarf- 

 shrubs that during winter the starch is replaced by fat, 

 the amount of fat increasing from October or November 

 and reaching its maximum in January or February, then 

 during spring it disappears entirely in temperate regions; 

 this is at least the case as regards a great many Danish 

 species. On the other hand, there are a number of species 

 in which no fat is visible during winter; as an example, 

 the food-storing roots in Pot. anserina may be mentioned. 

 While, as mentioned above, fat is absent from the deposi- 

 tory organs of the herbs during summer in Denmark, I 

 have, in P. nivea, P. emarginata and especially in P. pul- 

 chella (from N. E. Greenland) found fat in rhizomes and 

 roots throughout the whole summer; this is probably due 

 to the comparatively low temperature during summer in 

 Arctic regions. 

 F. A great many of the species have leaves which in one 

 way or other illustrate xeromorphy. Transpiration is 

 lessened (a) by a hairy covering: in several of the species 

 the under side of the leaf is densely matted or hairy, espe- 

 1 Indicated by use of Sudan III. 



