110 Knud Jessen. 



which the upper consists of high, narrow cells which enclose 

 large intercellular spaces. The cells in the second layer are 

 often very irregular; this layer is absent above the smaller 

 bundles in the marginal portion of the leaf. The proportion 

 between the leaf-thickness (about 220 n) and the thickness of 

 the palisade tissue is about \. Above the larger bundles the 

 assimilatory tissue is replaced by vein-parenchyma. The 

 spongy parenchyma is very loose in structure, the cells are 

 slightly or shortly stellately branched (Fig. 41, E). 



In the flower of Sorbus americana I have found about 

 15 stamens, of which the ten episepalous were only a little taller 

 than the five epipetalous. In Sorbus aucuparia there are twenty 

 stamens 1 . — In flower-buds which are on the point of open- 

 ing the stamens are found to be bent inwards over the 2 — 3 

 styles. The anthers are closed, but the stigmas are highly 

 papillose and apparently ready to receive the pollen. When 

 the flowers expand the stamens gradually straighten and 

 bend outwards towards the petals. I have not found open 

 anthers which were in contact with the stigmas, but they 

 are seen to be bending over the latter some time after they 

 have dehisced, and the pollen must almost certainly fall 

 upon the stigmas. In this homogamous stage the stigmas 

 were found to be covered with germinating pollen. 



In Greenland the flowering period occurs in July and 

 August, but according to Lange the plant does not flower 

 at higher altitudes. Rosenvinge thinks that the rowan sets 

 fruit regularly in favourable localities, and he gives some 

 instances of its having probably been dispersed by the agency 

 of birds. 



1 Cf. fig. 422 in Eug. Warming: Froplanterne (Spermatofyter), 

 København, 1912. 



