124 THISTLE, CENTAUREA. [CHAP. 



tubular, portion of the floret has a length of 3^ to 4 

 mm. ; the bellshaped portion, only of I I \ mm. The 

 flower heads have no ray flowers, and being therefore 

 much less conspicuous than the allied species, are 

 rarely visited by insects. 



Carduus arvensis (Cirsium of some authors) is the 

 commonest of our thistles. Each head contains 

 about 100 florets. The tube of the florets is 8 12 

 mm. in length, the upper part forming a bell-shaped 

 reservoir i i^ mm. in depth, with five diverging 

 linear lobes. As the lateral florets turn outwards, the 

 whole form a flower head, as much as 20 mm. in 

 diameter. Being therefore very conspicuous, and as 

 the honey in this species and most of its allies rises 

 into the cup of the flower, so as to be accessible even 

 to insects with very short tongues, it is visited by a 

 large number of species. Miiller records no less than 

 88. In C. lanceolatum, on the contrary, though it is 

 also a very common species, still in consequence of 

 the cup being somewhat deeper (4 6 mm. against 

 I i^ in C. arvensis}, and the honey therefore rather 

 less accessible, he only records twelve. In C. palustris 

 the depth of the cup is intermediate between those 

 of the two preceding species, and also the number of 

 insect visitors, namely 22. 



Onopordon differs from Carduus only in the 

 character of the receptacle, which does not bear 

 chaffy bristles, as in that genus. 



The genus Centaurea offers several interesting 

 points. In C.jacea, which is sometimes, for instance 

 by Bentham, regarded as a variety of C. nigra (the 

 Knapweed), 60 100 florets are united into a head ; 



