THE MIGRATION OF HOUSE-LEEKS 101 



illustrated, is characteristic of many Alpines. The 

 cushion plants, p. 179, and the carpet plants, are equally 

 compact, though entirely different in habit. The 

 compactness of the colony, cushion or carpet, tends to 

 reduce to a minimum the risk of intrusion of other 

 plants into the colony. 



In the photographs of Sempervivum arachnoideum 

 and S. montanum, on Plates XVIII., Fig. 1, and 

 XXIV., Fig. 1, it is obvious that room for the 

 extension of the colony can be found on its margins. 

 The colony can advance and overwhelm the present 

 occupants of the soil. Let us, however, examine 

 cases where space is restricted, and see how the 

 plant meets the difficulty. 



The photographs on Plates XXII. and XXIII. 

 show two colonies of the Mountain Sempervivum 

 growing in the crevices of an old wall bounding a 

 meadow near Saas Fee. For a time the plants have 

 been quite at home, but now the necessity for further 

 space to accommodate the growing colony has become 

 pressing. We notice that the colony is no longer 

 compact. We can now see the runners, which are 

 very much longer than they are under normal circum- 

 stances. Each bears a few small leaves, and ends in 

 a rosette-bud or offset. The runner arises in the axil 

 of a leaf of the parent rosette. 



We notice in the photograph on Plate XXII. 

 the crowded nature of the rosettes, and, further, 

 that the whole colony is tilted upwards on its side 

 to face the light. For this reason, some of the 



