196 
WILLIS - PODOSTBMACBÆ 
The seeds are usually very numerous, but in Farmeria we 
have a genus with a very small number (two or four) of 
larger seeds. 
Th@ ef t§^@ 
Not merely is it extremely hard to place the order in its 
proper position in a natural system of classification, but it is 
almost equally difficult, in the present state of our knowledge, 
to divide it satisfactorily into sub-orders, genera, and species. 
The whole physiognomy of these plants is so different from 
that of most Phanerogams, the morphology is so peculiar, the 
organs are so variable, the plants are so exposed to injury by 
water currents, and the condition of the vegetative plant 
below water is so different from that of the dry fruiting 
specimen above, that all authors who have worked only 
with herbarium specimens or with small quantities of 
material have fallen into errors in dealing with them. The 
study of these plants is beset vdth innumerable pitfalls for 
the unwary, and in no group is it so absolutely necessary to 
base distinguishing characters upon the examination and 
comparison of large quantities of material, and to study the 
entire life-history and morphology before diagnosing the 
groups. In numerous instances separate species and even 
genera have been founded upon different phases in the life- 
history of one plant, upon different parts of the same plant, 
or upon material taken from different levels upon the rocks, 
e,g.^ the genera Lawia and Terniola above-mentioned are 
separated upon the morphology of their thalli, one being 
interpreted as of root, the other as of shoot, nature. In 
reality the plants examined are only varieties of one species, 
agreeing in all essential details, but in the one case the 
centre, in the other the growing apex, of the thallus was 
examined. The cases of Dicræa pterophylla (= Wallichii) 
and Podostemon acuminatum have been already mentioned. 
The genus Hydrobryum as re-defined by Weddell is based 
on the fan-like stigma of the species H. Griffithii, the only 
species left in the genus by that author ; but if a number of 
