432 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
have a peculiarly interesting case, the two seeds remaining 
inside the fruit, which is held firmly against the rock, so 
that germination takes place under the most favourable 
conditions for the successful growth of the young plant. 
The dorsiventrality, dwarfing, &c., of the mature plants 
seem to show correspondingly in the seedlings, those 
showing them most early which have them most marked 
when mature, but we require a detailed comparative study 
of the family in this respect, such as Ganong has given to 
the Cactaceæ.* 
Anatomy. 
A number of anatomical features have been mentioned or 
described in the detailed part of the paper. Much also has 
been described by Warming, but there is yet need for much 
comparative investigation of these very curious plants, 
owing to the unusual features presented by the anatomy, in 
consequence of the far-reaching dorsiventrality of the vege- 
tative organs. Attention may be called to the structure of 
the thalli of Lawia, Podostemon, Hydrobryum, Dicræa, &c., 
described above, to the presence of silica bodies, and to the 
peculiar growth in thickness of the vascular tissues in the 
thallus of Dicræa and of the cortex of the thallus in the 
same genus, as well as to the peculiar root-caps, especially 
the remarkable rim-like cap of Hydrobryum olivaceum. 
In Dicræa and to a less extent in other genera, considerable 
anatomical changes take place at the time when the thallus 
is commencing to develop the flowers, owing to the 
demands made by the floral shoots for supplies of water and 
materials, especially when they are exposed to the air and 
begin to ripen their fruits. It is hoped to follow the present 
paper by others dealing with anatomy and other interesting 
features of many of the genera. 
Owing to the peculiar habitat of these plants, and the 
rigidity given to the creeping thalli by the presence of so 
much silica, it would seem not unlikely that such genera as 
* Ann. of Bot., 1898, p. 423. 
