MORPHOLOGICAL METHODS 
29 
then their ontogenetic phases will be represented roughly by 
abcdeFG H K L and abcdeMN 0 P Q, the larger 
type and spacing indicating greater distinctness and duration. 
Like other methods, development is most valuable in the 
investigation of recent phylogenetic changes 1 . 
Seedling Stages : Reversions : Polymorphism. 
In plants which when mature differ in morphological habit 
from their related forms, interesting phases of successive 
metamorphosis are often shown in the seedlings ; this is 
simply a special case of the ontogenetic recapitulation above 
mentioned. Good instances are Acacia, Bossiaea, Ulex, 
Hakea, many succulents, water-plants, &c. ; see also 
Retinispora, and cf. expanding buds of Acer, Aesculus, 
Berberis. In many respects the characters of seedlings 
are of special importance in comparative morphology. 
Single branches of mature plants at times show similar 
transitional or apparently ancestral phases of structure, 
e.g. in Acacia, Russellia, &c. Care must be taken, by 
employment of comparison and other methods, to confirm 
any deductions from such phenomena. If confirmed as 
probably ancestral, we may call them reversions or ‘ returns ’ 
to the ancestral type ( atavism ) ; if not confirmed by other 
evidence, they may be sports or monstrosities (above), or 
may be cases of dimorphism or polymorphism . Many organs 
appear in two or many forms on the same plant or on 
different plants of the same species; instances are the 
dimorphic shoots of Hedera and other climbers, leaves of 
many epiphytes and water-plants (Chap. III.), unisexual 
flowers, heterostyled (Lythrum) or right and left handed 
(Saintpaulia, Exacum) flowers, the flowers of Catasetum, &c. 
Multiple Origin and Parallel Descent are phe- 
nomena which appear to have been common in plants, and 
which increase the difficulties of morphological as of 
taxonomic study. Organs may appear alike in position, 
structure, function, and mode of origin, and yet not be 
phylogenetically homologous. Thus the leaves of mosses 
are not homologous with those of Selaginellas, having 
a totally different ancestry. In the case of adventitious 
1 And see Goebel, Organography of Plants, and Entwicklungs - 
geschichte der Pjlanzenorgane (Schenk’s Handbuch der Bot.). 
