Tubiflorce. 
*99 
notable for the constancy of the alternate leaf-arrangement, while 
most Apocynaceas have opposite leaves; but the exceptions to the 
latter are by no means rare. The aestivation of the corolla in Con- 
volvulacese is invariably contorted, as in the typical apocynal flower. 
Taken together, such considerations seem to point with no mean 
force to the conclusion that Convolvulaceac are to be regarded as 
the relatively near descendants of the apocynal plexus—that is to 
say, of the progeny of the apocynal stock typified in various Contorts. 
The fundamental advance which has been made upon the latter 
consists in the reduction of the ovules to a constant two per carpel. 
This reduction we have already recognized as a tendency in the 
Contorts ; the Convolvulaces represent the complete realization 
of this tendency; and with this has been coupled the tendency to 
the herbaceous habit,—a general feature of the Tubiflors {supra). 
Before the realization of this ovule-economy tendency had 
proceeded very far, however, the main line of tubifloral evolution— 
the line of zygomorphy—emerged (see diagram, p. 300). Very low 
down upon this line a side branch appeared, determined largely by 
the complete adoption of the herbaceous habit, the aggregation of 
the flowers into dense scorpioid inflorescences, and the unilocular 
condition of the ovary ; this line has the Hydrophyllaces as its 
terminal point. 
A short distance further along the zygomorphy line,—in other 
words, before the tendency to zygomorphy was realized to any 
great extent—a second side-branch appeared; this branch, terminated 
by the Solanaces, is to be regarded as a relatively short one, in 
view of the close connection between many typical Solanaceae and 
the Scrophulariaceae; the greatest divergence, indeed, is in the 
genus Solatium itself, with its very constant and somewhat specialized 
type of flower. This line of zygomorphy, notable for its early 
emergence from the ovule-reduction tendency line, will be traced 
ultimately to those multiovulate Tubiflorse in which the tendency 
to zygomorphy is realized— i.e, the Personates of Bentham and 
Hooker; these latter it will be convenient to refer to as Multi- 
ovulates. At some distance along the ovule-reduction line a branch 
emerged, leading to the Nolanacese, and determined by a tendency 
with which we shall deal at some length later {infra, p. 303)—the 
tendency to schizocarpy. 
Near the terminal point of the ovule-reduction line, when its 
tendency was almost fully worked out, so to speak, a second line of 
zygomorphy arose, to be terminated ultimately by the zygomorphic 
