PROTECTION OF POLLEN. 



117 



irCma acaulis, its large heads of flowers are used as hygrometers and weather- 

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

 md out in rays dry weather and a clear sky are indicated, but when the 

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

 How cone, wet and cloudy weather is anticipated (cf. fig. 224). The significance 

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

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

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

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

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



rig. 224.— Protection of Pollen. 



pitula of the Carline Thistle (Carlina acaulis), the one on the right open as in the sunshine, that on the left closed as at 

 night or in bad weather. 



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

 e honey also load themselves with the exposed pollen and subsequently convey 



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



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

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

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

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



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

 wards, unite into a compact tent, oflF which the drops of rain roll without being 

 lie to do any mischief. 



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

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

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



