Origin and Development of the Composites. 27 
type receives confirmation in the tendency to complete abortion of 
the corolla in the female florets of several of the genera showing 
the filiform type, namely Erigerou, Conyza, Hanstia (Fig. 9, 16-18), 
Psindia, Baccliaris (Astereae), and Gnaphaliuvi (Inuleae). This 
tendency is also shown by Heterothalamus in the Astereae, Clibadium 
and Iva in the Heliantheae, Doronicum and Petasites in the 
Senecioneae, and Leria in the Mutisieae. Apart from their bearing 
on the origin of the filiform corolla, these cases serve to emphasise 
the tendency to reduction of the corolla material which is general 
in the family. 
This tendency towards reduction is one of the critical tendencies 
which Wernham (90) regards as “ no less important than critical 
characters.” In considering the Campanulatae, however, Wernham 
noted the tendency to aggregation of the florets and apparently 
concluded that reduction followed in the relation of effect to the 
cause (aggregation). It has been pointed out by the writer (77, p. 30) 
that while the tendency to aggregation may be the primary cause 
of reduction, the reduction would act as, at least, a partial cause 
in subsequent aggregation, and that there has probably been a 
continuous aggregation and reduction mutually interacting until the 
limit of efficiency has been reached. 
Examples of the imperfect development of these two tendencies 
occur in the Lobelioideae, Dipsaceae, Valerianaceae and Umbelliferae. 
The development along these lines has proceeded under the 
pressure of orthogenesis beyond the limit of full efficiency in quite 
a number of Composites, such as the dense aggregations of capitula, 
which are frequently few flowered, in the Gundeliinae, Angianthinae, 
Relhaniinae, Echinopsidinae, Lychnophorinae. Lagasceinae and 
Milleriinae. It should be noted that these specialised sub-tribes are 
all shown to be end groups by the structure of the pollen-presentation 
mechanism (see Fig. 7, Chap. II). 
C. Development of the Corolla. 
For an account of the development of the complete floret the 
reader is referred to the previous publications discussed in Section 
A. The differences which characterise the mature corollas arise at 
an early stage. The difference in development lies in the behaviour 
of the two posterior petals. In the tubular corolla these two petals 
grow at the same rate as the others and remain in organic 
connection, as in Senecio (Fig. 9, 25, 26, 27), the mature floret 
showing a cross section, as in Fig. 9, 19. In the ligulate corolla 
