Tubiflorce. 
iS7 
corolla will be strongly zygomorphic and of elaborate structure; 
the stamens will number two only, with no trace of staminodes; 
each anther will possess but one fertile pollen-sac, the other being 
modified for purposes of attraction, pollination-mechanism, etc.; 
the bilocular ovary will contain two ovules in each loculus ; and the 
two-valved, explosive capsule will contain four large compressed 
seeds, each carried upon a woody, hooked process. Forms embodying 
most of these characters are common among the higher Acanthaceas 
(e.g. Justicieae). 
The Acanthaceae represent the terminal point of their branch 
of the evolutionary tree ; we have now to consider their points of 
contact with less advanced groups. The Transitional Group is 
recalled in the Thunbergieae, in which many characters of an 
ancestry which produced Convolvulaceae are preserved. The 
members of this section, unlike the rest of the family, are mostly 
twining plants, and include about 100 species. The flowers are 
typically large, with sub-regular corollas contorted in aestivation ; 
the ovary contains two ovules in each of its two loculi. In Mendoncia, 
with twenty-five to thirty species, the fruit is a berry and there are 
no retinacula; and in Thunbergia, with a capsular fruit, the 
retinacula are represented by cupular structures which are funicular 
in position and apparent origin, becoming horny at maturity. The 
resemblance between this section and Convolvulaceae is remarkably 
strong, both superficially and in detail. But the anisostemonous 
andrcecium (four stamens) and the primitive retinacula of Thun¬ 
bergia justify its inclusion in the higher family. 
A second small section, Nelsoniece, represents the link with 
Scrophulariaceae; the ovules are indefinite and the retinacula 
represented by small papillae. A third section, Ruellieae, may be 
regarded as a final transition to the higher Acanthaceae ; here the 
ovules are indefinite but few, with typical retinacula ; the stamens 
usually number four, and the anthers are rarely modified to any 
extent. 
The families with which we have dealt so far have all been 
regarded as offshoots from the scrophulariaceous stock. We now 
come to deal with a group which seems to approximate more closely 
to the remoter Multiovulatae-stock (see diagram, p. 152) in so far 
as it retains unmistakeable traces of the characters of the apocynal 
plexus (see chapter V). The chief family concerned is Bignoniaceae. 
These, unlike any family of Multiovulatae so far considered, are 
usually trees or shrubs, very often climbing plants, very rarely 
