46 Verr Brecuer Wirrrock. 
and so on. What has now been said may suffice to show that the 
differences in size within the group also give specific characters, though 
of a more subordinate kind. 
VII. ON THE GEOGRAPHICAL DISTRIBUTION. 
The Pithophoracee, being alge, are principally aquatic plants. 
Six of the species (eight in number) which are as yet known have 
been found in water. One, P. Cleveana nob., has been found on land, 
viz. on humid earth in the shade of bushes. How it is with P. Zelleri 
(v. Mart.) nob. can not be decided with certainty from the information 
which I have regarding it. It is said of this species, that it grows in 
rice fields, but whether in water or on wet earth is not said. 
It has already been mentioned that the aquatic Pithophoracee 
grow only in fresh water. Not one is known from wholly salt water, 
and only of one, the australian sterile form existent in the Grunowian 
collection, it is said that it occurs both in brackish and fresh water. 
With the exception of P. kewensis nob. all the species of this 
order have been found in warmer climates. By far the greatest part 
are even of a tropical origin. This is the case with P. suwmatrana 
(v. Mart.) nob., P. wqualis nob., P. oedogonia (Mont.) nob., P. Cleveana 
nob., P. polymorpha nob. and P. Roettleri (Roth) nob.; P. Zelleri (v. 
Mart.) nob. belongs to a subtropical climate. That P. kewensis nob., 
which has been found in England, also draws its origin from warmer 
countries, may be regarded as almost quite certam. This supposition 
is powerfully supported by the nature of its locality, which is, as has 
been mentioned in the introduction, the Tropical Aquarium or so-called 
Waterlily-house belonging to the Botanical Garden at Kew. The species 
here grows together with tropical Nymphwacee and other tropical aquatic 
plants in water which is always kept at a comparatively high degree 
of warmth. According to my researches it does not grow in the other 
aquaria at Kew, nor in the ponds and small lakes belonging to the 
garden. With a very great probability, as it seems to me, we may 
therefore conclude that spores of P. kewensis nob. have been brought 
with the rhizomes of the Nymphwacee or other aquatic plants from some 
tropical country, and that they have afterwards, when they have found 
circumstances advantageous to their development in the aquarium, ger- 
minated and brought forth specimens of the Pithophora capable of pro- 
pagation. If we suppose P. kewensis nob. also to have a tropical origin, 
