SALICINEAE 389 



are abundantly visited and pollinated by these insects. Lundstrom (' Krit. Bemerk, 

 ii. d. Weiden Nowaja-Semljas ') asserts that the pollen of northern species of Salix is 

 not so sticky as that of others owing to the possession of fewer droplets of oil and 

 is therefore easily dispersed by wind. But the presence of nectaries, even in northern 

 species, is against exclusive anemophily. 



Visitors. Herm. MuUer observed a solitary butterfly. 



2592. S. retusa L. (Herm. Miiller, loc. cit. ; Kerner, loc. cit.) In this species 

 also, according to Kerner, the female flowers mature some days before the male ones. 



Visitors. Herm. Miiller observed a single wasp. 



MacLeod (Flanders) observed, in willows in which the flowers appear be/ore the 

 foliage-leaves, the honey-bee, 3 humble-bees, 18 short-tongued Hymenoptera, 

 4 hover-flies, 14 other flies, 2 beetles, and a moth (Bot. Jaarb. Dodonaea, Ghent, 

 vi, 1894, pp. 131-3); ako, on species in which the flowers and foliage leaves appear 

 simultaneously, the honey-bee, 3 short-tongued bees, 6 hover-flies, and 3 other 

 Diptera (op. cit., p. 133). 



Schneider (Arctic Norway) observed the following 9 humble-bees on S. nigricans 

 Sm., S. glauca Z., S. Lapponum L., and S. phylicifolia Z, 



I. Bombus agrorum F., var. arcticus Acerbi; 2. B. alpinus Z.; 3. B. hypnorum 

 Z. ; 4. B. lapponicus F.; 5. B. nivalis Ddlhb.', 6. B. pratorum Z. ; 7. B. scrim- 

 shiranus K. ; 8. Psithyrus quadricolor Lep. ; 9. P. vestalis Fourcr. 



809. Populus L. 



Flowers anemophilous ; dioecious. Pollen scattered, according to Kerner, as 

 in Juglans. 



2593. P. nigra L. (Warnstorf, Schr. natw. Ver., Wernigerode, xi, 1896.) 

 In this species the yellowish bracts of the male spikes are glabrous at the margin, 

 slightly hollowed at the back, and provided with a low swelling round the edge, 

 which makes it difficult for the pollen to slip down. The pollen-grains are pale- 

 yellow in colour, irregularly polyhedral, tuberculate, 30-40 /u, in diameter. 



Visitors. Burkill (Yorkshire coast) observed the Muscid Onesia cognata Mg., 

 searching for nectar on $ flowers (' Fertlsn. of Spring Fls.'). 



2594. P. tremula L. (Sprengel, ' Entd. Geh.,' p. 439.) 



Visitors. Sprengel saw the male flowers of this species visited on the 15th ot 

 March, 1790, at Potsdam by numerous pollen-collecting honey-bees; 'The male 

 trees could be recognized and distinguished from the female ones at some distance 

 by the loud buzzing of these insects.' 



2595. P. pyramidalis Salisb. (Herm. Muller, ' Weit. Beob.,' II, p. 211.) 

 Visitors. Herm. Muller (Thuringia) observed thousands of po-cltg. honey-bees. 



CIV. ORDER EMPETRACEAE NUTT. 

 810. Empetrum L. 



Dioecious, sometimes with protandrous hermaphrodite flowers. Warming 

 describes them as anemophilous, but Lindman as entomophilous. The latter 



