PROTECTION OP POLLEN. 



117 



Carlina acaulis, its large heads of flowers are used as hygrometers aud weather- 

 ghisses. When the dry bracts surrounding the tubular florets of the capitulum 

 stand out in rays dry weather and a clear sky are indicated, but wlien the 

 liygroscopic bracts become erect and subsequently converge, so as to form a 

 hollow cone, wet and cloudy weather is anticipated {cf. fig. 224). The significance 

 of these movements of the radiating bracts or involucral leaves to the plant itself 

 is as follows. By day when the air is warm and dry the rays have an outward 

 curve and are spread out widely so as to turn their inner surfaces, which are 

 silvery white, to the sky, and they glisten so brightly in the sunlight that they 

 are visible from a great distance. They thus act as a means of alluring insects 



Fig. iH liMir< li 'h .'i I'oUen. 



Capitula of the Carline Thistle {Carlina acatdis), the one on the rif^lit open as in the sunsliine, that on the left closed as at 

 night or in bad weather. 



to the inconspicuous tubular florets of the disc, and these visitors whilst sucking 

 the honej^ also load themselves with the exposed pollen and subsequently convey 

 it to other flowers. A large number of humble-bees alight on the open capitula 

 of the Carline Thistle, suck the honey from the florets, and at the same time 

 remove the pollen. If at that moment there were to be a sudden shower of rain 

 the florets of the disc would inevitably be wetted and the pollen ruined. But 

 owing to their hygroscopic sensitiveness the rays rear themselves up on occasion 

 of even a slight increase of moisture in the air such as precedes rain, and, bending 

 inwards, unite into a compact tent, oflf which the drops of rain roll without being 

 able to do any mischief. 



Alterations in the form and position of certain tissues of the stamens due to 

 the taking-in and giving-out of water also aflford a means of protection for pollen 

 against wet in the case of Plane Trees, and of many Conifers, Yews, and Junipers 



