j.4‘2 James Small. 
at high altitudes in the Andes. The palseobotanical evidence shows 
that the origin of the family took place at approximately the same 
date as the first upheaval of the Andes. This synchronising of 
events in climatic evolution with events in plant evolution is in 
accordance with Coulter’s explanation of orthogenesis (see XI, 13, 
and Chap. XI, B). The rapidity of dispersal, which is character¬ 
istic of the family, would readily account for the appearance of 
representatives in the Mediterranean region within a geologically 
short interval of the origin of the earliest forms. The time 
available for this dispersal and the differentiation of the types is 
nearly the whole of the Eocene Period and opinions concerning even 
the approximate length of that period vary very considerably. 
It is more than probable that the Composite were among the 
first arrivals in the new mountainous habitats (cp. Krakatau and 
Taal, Chap. X), and this would account, in part at least, for their pre¬ 
dominance in those regions at the present time. The Compositae, 
indeed, seem to have been formed with the mountains by the 
mountains for the mountains. 
B. Cytology of the Composite. 
This part of the subject has been worked out tn considerable 
detail but only in a comparatively small number of genera. The 
chief papers will be found mentioned in Section B of Bibliography 
XII. Parthenogenesis, apogamy and double fertilisation have 
claimed attention but such data have little phyletic value, except 
as an explanation of the large numbers of microspecies in some 
of the genera. Spermatogenesis and oogenesis have been 
described in a number of forms and the figures given of Senecio 
vulgaris may be regarded as typical. 
Spermatogenesis :—Considering spermatogenesis first, the 
archesporium is sub-epidermal, usually only one row of cells 
(Fig. 68), which cuts off a single layer outside (Fig 69). This 
layer divides into three (Figs. 69-70); the innermost of the 
layers forms the tapetum, the cells of which are frequently 
bi-nucleate (Figs. 70-73). The middle layer finally degenerates 
and functions as a subsidiary tapetum (Fig. 73). The single row 
of archesporial cells rarely undergo more than one division 
before the pollen mother cells are formed (Fig. 70), so that 
only one tetrad of microspores is usual in tranverse sections of 
the sac (Fig. 73). The separation of the spore mother cells from 
each other takes place between synapsis (Fig. 70) and the meta¬ 
phase of the reduction division (Fig. 71). The differentiation of the 
exine has already been mentioned (Chap. IV, B) as being some¬ 
times intermediate in Senecio between the Tubuliflorae and the 
