ADAPTATIONS TO EXTERNAL MECHANICAL FORCES. 081 



A central or disc-floret of the Cornflower (fig. 294) illustrates these 

 features remarkably well. Besides the elasticity of the filaments, they 

 are very excitable just when the pollen is about to be shed. If a filament 

 be only touched it contracts, and then the excitation spreads to the others, 

 so that the anther-tube is depressed. It has been observed that this 

 shortening of the filament closely resembles the contraction of muscle, 

 though the latter will shorten by a third of its length, but a filament 

 only by one-sixth. This shortening is also effected by a discharge of 

 water from the cells into the intercellular spaces, just as has been 

 explained above in the case of the leaves of Venus' Fly-trap and the 

 Sensitive Plant. 



In the case of Stylidium, in which the filament and style are united. 



J,. 



Fig. 295.— Flower of Stylidium, show- 

 ing the column (filaments and style 

 united) bent down before irritation. 

 (After Burdon Sanderson.) 



Fig. 294. — Stamens and style of a 

 floret of Cornflower, showing con- 

 tractile filaments (&), below the " 

 anther-tube (a), arising from ihe 

 corolla-tube (c). (After Burdon 

 Sanderson.) 



the column at first bends downwards ; but if touched it springs over to 

 the opposite side of the flower (fig. 295). 



•These are but a few examples of quick-moving spring-like structures 

 in flowers. In all cases the use of such displays of mechanical force is to 

 secure pollination by insect visitors. 



The Orchid family supply several very interesting cases ; in some it 

 is the labellum which moves, in others, as Catasetum, it is the pollen- 

 mass and its appendages which constitute a powerful spring ; but as 

 these have been described very elaborately by Darwin in his Avork 

 ^' The Fertilisation of Orchids," I need only refer the reader to it. 



The stigmas of flowers are sometimes movable. Thus, in the Musk 

 plant and other species of Mimulus, the two stigmas are like flaps 



