Small, — Pollen-presentation Mechanism in the Compositae. 459 
this is a very uncertain guide to relationship ; for example, the present 
writer ( 18 ) and also Schmidt ( 16 ) and Hoch ( 9 ) have studied the hairs of 
the families of the Tubiflorae, and it is found that, while some allied 
species or even genera may show very similar hairs in many cases, allied 
species may have hairs which are quite dissimilar. The study of the 
achenial hairs of Compositae leads to the same conclusion. Vuillemin ( 20 ) 
showed that anatomy was no certain guide to affinity, and later papers 
(8 and others) confirm his conclusions that anatomical characters are reliable 
only in certain special and very limited groups. The floral characters, 
therefore, become of supreme importance, and as they are so uniform 
throughout the order, the value of details in their structure must be con¬ 
sidered carefully. Linnaeus naturally classified the group according to the 
condition of sexuality in the florets, but the first systematist to consider the 
matter in detail (i. e. Cassini ( 3 )) saw at once that the dominating character 
was the pollen-presentation mechanism, and he accordingly gave the 
greatest value to the characters of the styles and stamens. Lessing ( 13 ) 
followed Cassini, but laid more stress upon the characters of the style in 
the delimitation of the larger groups, and in this he is followed by 
Bentham ( 2 ). For generic distinctions the venation of the corolla was 
used to some extent by Don, and the form of the achene by Schultz 
Bipontinus ( 17 ). 
Although the value of the details of structure of the styles and stamens 
has thus been recognized for purposes of classification, the significance of 
these details to the plants themselves has been considered negligible. Of 
all recent synantherologists, Bentham gave the highest value to the variation 
in the appendages of the stamen, but he writes ( 1 ) : ‘ The anthers, however, 
are sometimes provided with certain appendages apparently of little or no 
functional or homological importance, but which, nevertheless, from the 
remarkable constancy of their presence or absence in whole tribes, supply 
one of the most valuable characters in Compositae if applied with proper 
caution.’ 
Since the highest development of these appendages is found in the 
same groups with the highest complexity in the structure of the style, it 
becomes highly probable that the appendages have quite a definite function, 
and a consideration of the facts shows that here, as in the rest of the 
characters, economy is the dominant factor. Economy of corolla material 
leads to the aggregation of the flowers into a capitulum ; economy of calyx 
material leads to the entire disappearance of that part of the flower or to 
its modification into a pappus ; economy of stamen material leads to reduc¬ 
tion in number of the stamens to the minimum compatible with the 
efficiency of the pollen-presentation mechanism ; economy of carpel 
material leads to reduction in the number of carpels to two, and of ovules 
to one. From this it will be seen that the polliniferous tissue is the only 
H h 2 
