Camp emulate? . 295 
this arrangement is clear. The dense aggregation of the flowers 
permits of the pollination of several at a single brief visit from an 
insect—and this operates in the way of compensation for any dis¬ 
advantage accruing from the presence of one ovule only in each 
flower. The high degree of certainty of pollination has been 
indicated earlier in this chapter. Lastly, the advantage of a fruit- 
body for each individual seed (chapter V) is secured, and the 
calyx, needed no longer for the purposes of protection is utilized 
for the important function of dispersing this fruit, in virtue of its 
modification into a pappus ; a similar condition has already been 
described for the Dipsacales (chapter VI1). 
The composite head, from the aspect of insect-attraction, is the 
biological equivalent of a single flower. This condition results 
primarily from the development of a “ ray ” senes of florets, usually 
the product of extreme zygomorphy, sometimes the mere increase in 
relative size, of the exterior members of the inflorescence ; the latter 
are usually unisexual (female) or in some cases quite sterile, as in 
Viburnum, noticed in the preceding chapter. The zygomorphy is 
typically of that peculiar kind expressed in the so-called ligulate 
floret; and in one extensive section of Compositae, Liguliflorae, 
generally regarded as including the most advanced genera, zygo¬ 
morphy has invaded the whole capitulum, which consists entirely 
of ligulate florets. 
It is worthy of note that, given the cohesion of the anthers to 
form a tube, tbe actual mechanism of the pollen-presentation in 
Compositae and Campanulaceae amounts to nothing more than the 
growth and development of a part of the flower, the style ; and 
the mere simplicity of this arrangement is doubtless much in its 
favour from the standpoint of biological advantage, in contrast 
with the subtleties of specially-developed organs like the “ pollen- 
cup” of Goodeniaceae and the irritable gynostemium of Candol- 
leaceae. 
Referring again to the evolutionary tree of Campanulatae, 
(diagram, p. 230), it will be gathered that the line of aggregation is 
clearly distinct from the line of zygomorphy in the individual 
flower; the two lines emanating from the Campanal Stock are 
thickened in the diagram. A transitional stage along the aggrega¬ 
tion line is reflected in a small family, Calyceracece, characterized by 
definite involucrate capitula and by the presence of a single ovule 
only in each floret: but there is no zygomorphy of the outer florets, 
nor is the calyx materially reduced. 
