THE HOUSE-LEEKS 99 



may be less pronounced, and the rosette appear 

 to be green and not white. On the other hand, 

 where fully exposed to the sun, the spider's web is 

 usually very much in evidence. 



The object of these matted hairs would appear to 

 be to bind together the leaves of the rosette as com- 

 pactly as possible, and thus ensure that each leaf is 

 held erect, and that the risk of excessive loss of water 

 by transpiration (p. 12) is reduced to a minimum. 

 The fact above mentioned, that, in shady places, where 

 there is less danger from evaporation, the web is much 

 less in evidence or is only feebly developed, supports 

 this view. 



The hairs of the spider's web, as Fraulein Dintel 

 of Vienna has shown, are modified glandular or 

 secretory hairs. Glandular hairs are abundant on the 

 leaves and flowering shoots of many Alpine species of 

 this genus. Fraulein Dintel finds that it is the secre- 

 tion of such hairs which forms the means whereby 

 they are bound together into the web. 



Another common House-leek to be found on 

 Alpine rocks is the Mountain House-leek {Semper- 

 vivum montanuni, Linn.) (Plate XXIV., Fig. 1, 

 Plate XXIL, and Plate XXIIL), which like the 

 preceding species has rose-coloured flowers but no 

 spider's web on the leaves, though they possess 

 ordinary, small glandular hairs. This plant is per- 

 haps the most abundant representative of the genus in 

 the Alps. 



We may take these two plants — the Spider's -web 



