102 ANGIOSPERMAE—DICOTYLEDONES 



stamens — which have dehisced introrsely, and are somewhat higher than the stigma — 

 and then the stigma with its opposite side, so that cross-pollination is promoted. 

 Crossing is subsequently rendered even more easy, for these anthers turn their 

 pollen-covered surfaces upwards. 



Visitors.— MacLeod observed — in the Pyrenees — a butterfly — Anthocharis belia 

 Cr., var. simplonia Freyer, skg. — adapted to the structure of the flower. 



236. S. alba L. (Hildebrand, Bot. Jahrb., Leipzig, xii, 1881, p. 26; Kirchner, 

 'Beitrage,' pp. 22-3.) — The crowded golden-yellow flowers exhale an odour like that 

 of vanilla. The claws of the petals are 5 mm. long and at first erect ; when the limbs 

 are expanded the diameter of the flower is 15 mm. The stigma, and the anthers of 

 the long stamens, project 2-3 mm. beyond the corolla. Automatic self-pollination 

 does not take place, however, for the anthers turn their dehisced sides outwards, away 

 from the stigma. The two short stamens are 3-4 mm. below the stigma, with their 

 pollen-covered sides facing inwards. Two of the four nectaries are internal to the 

 bases of the long stamens. 



Visitors.— On cultivated plants at Kiel I noticed the honey-bee, skg., and also 

 a hover-fly — Eristalis tenax Z., po-dvg. 



67. Erucastrum Presl. 



The yellowish flowers are homogamous or slightly protogynous, with exposed 

 nectar. Four nectaries. 



237. E. obtusangiilum Reichb. (Kirchner, 'Beitrage,' pp. 22-3.) — The sepals 

 of plants at Zermatt project horizontally, as in the case of Sinapis arvensis, so that 

 the nectar is accessible to insects from the outside. But the flowers are so 

 crowded that it is more easily reached from above. There are four functional 

 nectaries, of which two — according to VelenovskJ''s figure — are broad and flat, 

 and situated on the inner sides of the bases of the two short stamens, projecting 

 between the claws of the petals. The latter are 5 mm. long, upwardly directed, 

 and closely apposed laterally. When the limbs are fully expanded the diameter 

 of the corolla is about 12 mm. Before dehiscence the anthers have a dark red 

 spot at the tip. They all turn the dehisced side inwards, but are remote from 

 the stigma, which is simultaneously mature, and is placed just above the entrance 

 of the flower, so that when the flowers are in the erect position, automatic self- 

 pollination cannot usually take place. The lower ends of the anthers of the long 

 stamens are at the same level as the stigma ; those of the two short stamens are 

 somewhat lower. 



Visitors. — MacLeod observed — in the Pyrenees— 6 bees, 6 Lepidoptera, a 

 beetle, 5 Syrphidae, one of the Bombyliidae, and a Muscid. (' Pyreneenbl.,' p. 392.) 



68. Diplotaxis DC. 



The moderately large yellow flowers have a pleasant odour, and are homo- 

 gamous, with half-concealed nectar. Four nectaries. 



238. D. tenuifolia DC. (MacLeod, ' Untersuchungen u. d. Befrucht. einiger 

 phanerog. Pfl. d. belg. Flora,' II; Kirchner, 'Flora v. Stuttgart,' p. 301; Schulz, 



