CRUCIFERiE 



81 



twenty species have been observed, but this is probably 

 by no means all. I have given (ante, p. 76) the embryo 

 of this species as an illustration of an " incumbent " 

 embryo. The genus is named from the Greek hesperos, 

 evening, because it becomes specially sweet at night, 

 which generally indicates fertilisation by moths. In 

 support of this I may mention that H. Miiller's daughter. 

 Miss Agnes Miiller, saw several species of moths visiting 

 the allied species, H. tristis. No evening insects, how- 

 ever, are as yet recorded. 



The plant is a native of Central and Southern Europe 

 and Eussian Asia, except the extreme north ; it is found 

 in England only as an escape from gardens. 



Sisymbrium 



S. officinale (Hedge-mustard). — The plant is hairy, 

 and seems to have a peculiar affinity for dust. There 

 is a honey-gland on each side of the two short stamens. 

 When the flower opens, the pistil and the longer stamens 

 project slightly. The shorter stamens are enclosed in 



A B 



Fig. 47. — Sisy7nhrium offlcinale. A, Flower in the first stage. B, Flower in the 

 second stage, k, sepal ; c, petal ; ti, anther of one of the longer stamens ; a', 

 that of a shorter one ; a", stigma ; 7t, nectary. 



the flower, but the anthers are open. All six stamens 

 then grow a little so that the anthers of the shorter ones 

 reach the stigma, while those of the longer ones project 

 slightly beyond it. The flower is therefore adapted for 

 cross-fertilisation, but may also be fertilised by its own 

 pollen. It is visited by bees, flies, and butterflies. It is 

 a native of the north temperate zone of the old world, 

 and has been introduced into the United States. 



G 



