30 
POL YPHYL Y 
roots (see below) organs similar in structure, function, 
development, and position appear to have arisen in similar 
ways in various groups of plants, i.e. in various lines of 
descent or phyla ; in each group the roots are mutually 
homologous, but the whole class of “ adventitious roots” 
is polyphyletic , and the roots in different groups are only 
homoplastic , i.e. equivalent in structure and mode of origin 
on the plant, but not of common (though of parallel) 
descent. If we can also prove common descent they are 
of course homologous. Homoplastic organs must not be 
confused with analogous organs, which agree in function but 
not in descent nor mode of origin nor position, e.g. the 
root-like leaves of Salvinia are analogous to roots. 
As in taxonomic work (Chap. II.) so in morphological, 
care must be taken to work with the aggregate of all the 
characters of an organ and to use comparison extensively, 
otherwise these phenomena of multiple origin ( polyphyly ) 
and parallel descent ( homoplasy ) are liable to cause error 
and confusion 1 . 
Concrescence, or union of originally distinct organs 
by growth of the tissue beneath them, is a common 
phenomenon of structure. Organs in their earliest stages 
of development are usually separate, and most often remain 
so, but frequently there is a subsequent uniform growth 
of the tissue under them, 
carrying them out upon 
a basal portion which leaf 
most often (e.g. in sym- 
petalous flowers) has a 
similar structure to the i ea f 
organs themselves, so 
that they look as if 
joined together by their bases. The diagram shows this 
process on the right, but not on the left. It is common 
in flowers, less so in other organs. It is not necessary for 
the concrescent organs to be similar; e.g. stamens are often 
united to petals, leaves to stems (cf. Solanaceae, &c.). The 
phenomenon goes under many names — adnation, connation, 
adhesion, cohesion, &c. (see below). 
1 Bower, Presidential Address , Brit. Ass. Rep. 1898 ; Willis, 
Podostemaceae (§ on dorsiventrality), Ann. Perad. 1. 1902, pp. 41 1, 446. 
leaf 
leaf 
