196 BRITISH FLOWERING PLANTS chap. 



Lastly, of the yellow-flowered species, one {S. rupestre) 

 is over 6 inches high, the other two not above 3 inches, 

 one {S. acre) bitter, the other {S. sexangulare) taste- 

 less. The fleshy leaves check transpiration, as the 

 surface is, of course, in a smaller proportion to the 

 volume ; the succulent tissues also enable them to lay 

 in a store of water as a reserve in times of drought. In 

 some cases the character of the contained fluid is also 

 an impediment to transpiration ; gummy, saline, and 

 resinous fluids evaporate much more slowly than pure 

 water. 



It was in species of Sedum that Bohm in 1856 

 observed^ the movements of the chlorophyll grains 

 under the action of light. In difi"used light the grains 

 are scattered, while in bright light they collect into 

 heaps. He did not, however, give details, and the 

 phenomenon is more clearly shown in Oxalis (p. 135) 

 and Lemna. 



S. Rhodiola is dioecious ; according to Warming, in 

 Greenland tricecious. 



S. acre. — The flowers are protandrous. The five 

 outer stamens, those opposite the sepals, first stand 

 upright and open their anthers. When the pollen is 

 shed they curl outwards and make way for the inner 

 series, which take up the same position. When these 

 also have shed their pollen, they in their turn curve 

 downwards and outwards, while the five carpels grow 

 up, and the small terminal stigmas occupy the same 

 central position. Self-fertilisation ' would thus be ex- 

 cluded. H. Miiller, however, found in Westphalian 

 plants that the second series of stamens and the stigmas 

 ripened together. The bitterness, no doubt, as in other 

 cases, serves as a protection. 



S. Telephium. — In this species, on the other hand, 

 H. Miiller found that both sets of stamens have shed 

 their pollen before the stigmas come to maturity. 



S. album is strongly protandrous. 



^ Sitzungsh. Akad. JViss. IFien, xxii. (1S56). See also Frank, Frmgsh. 

 Jahrb. viii. (1872). 



