126 INTERESTING PLANTS OF ALPINE PASTURES 



a penthouse over the flowers on the approach of wet 

 weather, so as to protect the pollen (Plate XXIX., 

 Fig. 2). On sunny days the bracts bend backwards 

 on to the leaves, and leave the flowers fully exposed 

 to the heavens. 



When the flower-head is young and its diameter 

 relatively small, the length of the bracts is also 

 short. But as the flower grows and increases in 

 size, so the bracts also lengthen. Thus the roof or 

 penthouse grows in proportion with the dimensions 

 of the whole flower-head, and protection is at all 

 times efficient. 



It has been found experimentally that it is not a 

 change in the intensity of the Hght or of temperature, 

 which stimulates this Thistle to erect its penthouse, 

 but a variation in the amount of humidity or moisture 

 in the air. For this reason these flower-heads are 

 often used as weather-glasses or hygrometers. 



The heads of the Carline Thistle are wonderfully 

 conspicuous, especially when expanded, and this is 

 largely due to the silvery white, glistening, inner 

 sm^face of the strap-shaped bracts. The flowers of 

 the head are much visited by humble-bees, which carry 

 the pollen from one flower to the stigma of another. 

 The stylar-brush mechanism, already described on 

 p. 85, exists in this, as in all other members of the 

 Compositse. 



