246 BRITISH FLOWERING PLANTS chap. 



Centaubea 



Herbs, sometimes spiny. Eeceptacle with scales. 

 The bracts are often curiously and beautifully fringed. 

 The outer row of florets is generally without stamens 

 or pistil. The disk flowers are complete. Achenes 

 glabrous, generally with a pappus of short hairs. The 

 pollen - brush is not situated at the summit of the 

 stigmas, but forms a ring round the pistil just below 

 where it bifurcates. When the flower opens, the pollen 

 has already been shed into the hollow space between 

 the pistil and the hood formed by the anther-heads. 

 It is not, as in most Composites, pushed up by the 

 growing pistil, but the stamens are sensitive, as in the 

 previous genus, and when touched by the proboscis of an 

 insect they contract, thus exposing the pollen. This 

 interesting process can easily be seen if the stamens are 

 gently touched by a hair. The sensitive part appears 

 to be that covered by the hairs. If the stamen be 

 touched above or below the hairs no movement takes 

 place. ^ Eventually the stigmas open and curl over, so 

 that if insect visits are delayed the flower can hardly 

 fail to fertilise itself. We have six species. Two are 

 prickly, one, C. Calcitrapa, with purple, the other, 

 C solstitialis, with yellow florets. Of the other four, 

 two have the involucral bracts ending in small teeth ; 

 one, C. Cyanus, is bright blue ; the other, C. aspera, is 

 purple. The last two have the involucral bracts with 

 a broad black or brown fringed border, one, C Scahiosa, 

 with deeply pinnatifid leaves, while in C. nigra they are 

 nearly entire or toothed. The stamens contract to from 

 one-tenth to one-fifth of their length. They take about 

 ten minutes to return to their original state. They are 

 very elastic, and can be stretched to double their usual 

 length, contracting again to their former size when the 

 force is removed. The contraction appears to affect 

 the whole parenchyma, but not the spiral vessels, which 

 consequently are waved. The tension, however, must 



^ Haberlandt, Sinnesorgane im Pflansenreiche. 



