396 



ANGIOSPERMAEDICOTYLEDONES 





flower increases to twice its original diameter or even more, so that the visits 

 paid by any particular insect will usually be in the order most favourable for 

 crossing. 



We are indebted to A. Engler for an excellent monograph on the genus. 

 He regarded all the species as protandrous. Treviranus (Bot Ztg., Leipzig, xxi, 

 1863) had already observed the movement of the stamens towards the middle 

 of the flower, and inferred that automatic self-pollination takes place. As some 

 species of this genus possess nectar-guides, while others do not, Engler called in 

 question the correctness of Sprengel's interpretation of these markings. Hermann 

 Miiller remarks in this connexion ('Fertilisation,' p. 243), that in plants where the 

 nectar is sometimes fully exposed, and at other times concealed, there is very 

 naturally a corresponding variation in the nectar-guides, so that this peculiarity 

 of the genus cannot be regarded as opposed to Sprengel's view, while there is no 

 other explanation to take its place. 



938. S. Aizoon Jacq. (Herm. Miiller, ' Alpenblumen,' pp. 100-2.) The 

 flowers of this species are markedly protandrous, and in the Alps self-pollination 

 is almost or entirely prevented. The abundant pollen and readily accessible nectar 



Fig. 124. Saxifraga Aisoon,Jacq. (after Herm. Miiller). A. Flower at the beginning of the first 

 (male) stage. B. The same, at the end of the first stage. C. Flower in the second (female) stage (X3iX 

 , nectary. 



attract numerous visitors, especially flies. In Greenland, where insects are few, 

 the flowers are also strongly protandrous, but here effective automatic self-pollination 

 is ultimately possible by contact of the anthers (still covered with pollen) with the 

 widely divergent stigmas (Warming, Bot. Tids., Kjobenhavn, xvi, 1888, pp. 27-9). 



Visitors. Herm. Miiller observed (in the Alps) no fewer than 61 species of 

 flies (including 37 Muscids); also 5 beetles, 11 Hymenoptera, and 10 Lepidoptera. 



Loew saw the Syrphid Cheilosia modesta Egg.(i) in Switzerland ('Beitrage,' 

 p. 56). MacLeod noticed a short-tongued Hymenopterid and 5 Muscids in the 

 Pyrenees (Bot. Jaarb. Dodonaea, Ghent, hi, 1891, p. 420). 



Loew observed 2 Syrphids in the Berlin Botanic Garden 1. Ascia podagrica 

 J?., passing from flower to flower, hovering a considerable time over each, and then 

 alighting to suck; 2. Melithreptus scriptus Z., do. 



