378 ANGIOSPERMAE—DICOTYLEDONES 



Kirchner, ' Neue Beob.,' p. 12; Meehan, Bot. Centralbl., Cassel, xvi, 1883, p. 338; 

 MacLeod, op. cit., p. 124 ; Wehrli, Flora, Marburg, Ixxvi, 1892 ; Loew, ' Bliitenbiol. 

 Floristik,' p. 335.) — Several observers (Bail, Baillon, Newdigate, Schulz) have 

 noticed hermaphrodite as well as unisexual flowers in this species. Hildebrand 

 states that only young trees bear flowers, but according to Kirchner this is not 

 always the case. Meehan says that if the spring is warm the S flowers scatter their 

 pollen before the 5 ones are fully mature ; if cold, however, the stocks are homo- 

 gamous. In the first case few fruits are set, but in the latter they are abundant. 



Kerner asserts that pollen is only scattered in dry windy weather ; in other 

 circumstances it is stored inside the flower in a place sheltered from the rain. Alnus, 

 Betula, Populus, and Carpinus agree in this respect. 



Stocks may be homogamous, protogynous, or protandrous, in different places. 

 Kirchner found they were usually homogamous at Stuttgart, but the stigmas were 

 still receptive after the anthers had dehisced ; he also observed rare protandry. 

 Kerner describes the stocks as protogynous. MacLeod found them to be homo- 

 gamous in Flanders. Those which I examined in Kiel were protogynous to such 

 an extent that in some circumstances the interval between the maturation of the two 

 sexes might last a week {cf. Vol. I, p. 43). Wehrli observed a bush near Aarau, 

 which for two consecutive years bore only female flowers instead of male catkins, 

 and corresponded completely to the latter, except that the four stamens were replaced 

 by four stigmas, while ovaries were absent. Warnstorf describes the pollen-grains as 

 sulphur-yellow in colour when examined in the mass, tetrahedral, smooth, about 31 /a 

 in diameter, with three germinating processes. 



Visitors (to 5 catkins). — Herm. Miiller (Lippstadt, ' Fertilisation,' p. 523) and 

 Knuth (Kiel) observed the honey-bee. Burkill (Yorkshire coast) noticed the 

 Syrphid Melanostoma quadrimaculata Verral S and 5, po-dvg. (' Fertilisation of Spring 

 Flov/ers'). 



805. Carpinus L. 



Flowers anemophilous ; monoecious ; the 5 bearing numerous stamens and 

 the 5 arranged in loose spikes. 



2570. C. Betulus L. (Sprengel, 'Entd. Geh.,' p. 431; Kerner, loc. cit.; 

 Warnstorf, Schr. natw. Ver., Wernigerode, xi, 1896 ; MacLeod, Bot. Jaarb. Dodonaea, 

 Ghent, vi, pp. 125-6.) — The J flowers of this species are arranged in pendulous 

 spikes, the anthers borne on a connective with two arms of equal length, extrorse, 

 partially brownish-red in colour, possessing a long apical tuft of white hairs. The 

 pollen-grains are whitish-yellow in colour, irregularly polyhedral, tuberculate, on an 

 average 50 /a in diameter. MacLeod says that the S and $ catkins appear at the 

 same time as the leaves. The § are situated above the 5, but this is reversed in 

 the case of Betula and Alnus. 



806. Betula L. 



Flowers anemophilous ; monoecious ; occasionally hermaphrodite ; rarely 

 dioecious. Kemer describes the plants as protogynous, the female flowers maturing 

 earlier than the male ones of the same stock. 



2571. B. alba L. (=B. verrucosa Ehrh.). (MacLeod, op. cit., pp. 119-21 ; 



