622 ANGIOSPERMAE—DICOTYLEDONES 



the stylar branches gradually bend round till their stigmatic surfaces touch any 

 pollen that may remain, and are thus automatically self-pollinated. 



The florets develop in centripetal order (as in all Compositae), so that in the 

 disk of a fully mature head they may be observed in the following stages, beginning 

 at the outside, — faded, female stage, male stage, not yet open. As flowering 

 progresses inwards, the at first sharply arched common receptacle swells up and 

 becomes globular, so that florets which have yet to shed their pollen or are in 

 the act of doing so are at a considerably higher level than those that have faded. 

 It follows that alighting insects settle only on the former, and they avoid the faded 

 florets situated on the sloping surface below. The arrangement is favourable to 

 geitonogamy, as in the tj^pe form. 



Visitors. — I observed the hover-fly Eristalis arbustorum L., and the Muscid 

 Scatophaga merdaria F., both skg. 



1471. M. discoidea DC. (= Matricaria suaveolens Buck., and Chrysanthemum 

 suaveolens Aschers.). — In this species there are no ray-florets. The mechanism of 

 the yellow disk-florets appears to be the same as in the last two species. Warnstorf 

 (Verh. bot. Ver., Berlin, xxxviii, 1896) describes it as follows. — Many very small 

 hermaphrodite tubular florets about \ mm. long are aggregated into an almost 

 spherical yellowish-green head. They turn green after the pollen is matured, 

 and become larger, so that the head increases considerably in size. The pollen 

 is not raised above the corollas by the stylar branches, but lies between their lobes, 

 which protect it from being carried away. Owing to the smallness of the flowers, 

 geitonogamy inevitably results from the divergence of the stylar branches. The 

 pollen-grains are yellow in colour, polyhedral, with spinose tubercles, about 25 /«. 

 in diameter. 



Visitors. — I observed the hover-fly Syritta pipiens L., skg. and po-dvg., and 

 the Muscid Scatophaga stercoraria Z., skg. 



456. Tanacetum Toum. 



Ray-florets are wanting in some species, when present ligulate, and white in 

 colour. Otherwise like Matricaria. 



1472. T. vulgare L. (= Chrysanthemum Tanacetum Vis.). (Herm. MuUer, 

 'Fertilisation,' pp. 232-3; MacLeod, Bot. Jaarb. Dodonaea, Ghent, v, 1893; Knuth, 

 ' Bl. u. Insekt. a. d. nordfr. Ins.,' p. 91.) — The heads of this species are in one plane, 

 as in Achillea, conferring the advantage that insect visitors can creep over the whole 

 surface without having to use their wings, and a very large number of florets can be 

 pollinated at the same time. The crowding together of numerous heads, though 

 these are small and rayless, makes the plant so conspicuous that it is very easily seen, 

 and therefore visited by a large number of insects. 



Hermann Miiller states that each head contains several hundred yellow florets. 

 The bell of the corolla is only i mm. deep. The style has the same structure as 

 in Achillea : it bears a capitate bunch of diverging sweeping-hairs at the tip of its 

 branches, by which pollen is swept out of the anther-cylinder during the first stage of 

 anthesis. In the second stage the stylar branches, which are beset internally with 



