412 WILLIS : MORPHOLOGY OF THE PODOSTEMACBÆ 
which at present go under that name in the various groups 
of Pteridophyta, in the Gymnosperms, and in Monocotyledons 
and Dicotyledons, and include in the category not only the 
primary or embryonic roots, but also the roots which are 
adventitiously developed from stems or other parts, we must 
include in the general term the thalli of the Podostemaceæ. 
For the present this is no doubt the best course, just as it is 
still, and will probably long remain, best to continue using 
such classes as Fungi, Gymnosperms, Sympetalæ, though we 
now consider them as almost certainly polyphyletic ; until, 
however, our knowledge of the phyla is much more com- 
plete, it will be unsafe to venture far in proposing new 
classifications based thereon. Let us then accept the term 
root as the general classificatory expression for a group of 
organs which have certain features in common, but which 
have in great probability not all arisen from one phylum 
of descent. In this large group we must make many small 
ones, but it will be safer not to draw too many lines of con- 
nection through these until we know more of their actual 
phylogeny. Thus we may make classes for the roots 
derived directly from the embryo in each group of Pteri- 
dophyta and Gymnosperms, and probably may include in 
another the corresponding roots of Monocotyledons and 
Dicotyledons, the latter class probably to be united to one or 
more of the former. Secondary or adventitious roots, again, 
must apparently form another polyphyletic group, with many 
sub-groups, among which one must be kept for the thalli of 
the Podostemaceæ, another for the thorny roots of certain 
palms, and so on. 
Our conceptions of the morphology of the root require to 
be much enlarged, just as has happened in recent years in 
the case of shoots, sporophylls, &c. The root in general is so 
uniform in structure, owing to general uniformity of func- 
tion, and is so concealed from view and inconvenient to 
work with, that its morphology and other features ha^e 
tended to be neglected in favour of those of the subærial 
organs. 
