98 ROCK PLANTS OF THE PASTURES 



THE HOUSE-LEEKS. 



The House-leeks, genus Sempervivum (natural 

 order Crassulaceae, the Stonecrop family), are among 

 the most striking Alpine plants in dry rocky situations. 

 The leaves are thick, fleshy, or succulent, and arranged 

 in rosettes (Plate XXIV., Fig. 1) close to the ground. 

 The flowering stems also bear similar leaves, but these 

 are smaller and more scattered in their arrangement. 

 The leaves of the rosette are held erect that is to say, 

 the apex of the leaf points directly upwards and thus 

 the sunlight only falls obliquely on their surfaces. 

 This is another adaptation which tends to reduce the 

 loss of water given off by the leaves. The leaves have 

 also water-storage reservoirs in their tissues, and so 

 are well adapted to the dry barren soils on which they 

 live fully exposed to the summer sun. 



In Britain, one species of House-leek (Sempervivum 

 lector 'urn, Linn.) is common on cottage roofs and on 

 old walls. This plant is also not infrequent in 

 Switzerland. In the Alps there are also several 

 species with large, handsome, rose-coloured or yellow 

 flowers, which do not occur in Britain. 



The Spider's - web House - leek (Sempervivum 

 arachnoideum, Linn.) (Plate XVIII., Fig. 1) has 

 peculiar rosettes in which the tips and edges of the 

 leaves are all bound together by a white network of 

 long hairs, the whole meshwork resembling a spider's 

 web, hence the specific name (Plate XXL). Where 

 the plant grows in very shady places, the network 



