170 
On the History, Histological Structure , 
on Nematopliycus. His great skill in microscopic manipulation and 
observation has been of service to me in this, as in many other 
investigations. It must be remembered, however, that in this fossil, 
we are dealing with only a fragment of what must have been a 
very large organism. Perhaps the size may be thought to militate 
against the affinity of the fossil with the recent Algae. As I have 
before said, some of the specimens included under the names 
Prototaxites Logani are not even congeneric with the one which 
has been figured, and I believe those which I have excluded are the 
large stems on which Dr. Dawson’s statements of the great size of 
Nematopliycus are based. No doubt, however, the fragment 
exhibiting the structure described, belonged to a plant of consider- 
able size. But mere bulk is of little importance among Algae. 
Their increase depends simply on the repetition of similar vegetative 
parts, which in many recent species goes on to almost any extent. 
It may be mentioned that among the living Algae to which I 
have been referring, there are some remarkable phenomena in re- 
gard to vegetative bulk, if not of the organism as a whole, at any 
rate of the component parts. Take, for example, the extraordinary 
size of the single branching cell which forms the frond of the different 
species of Bryopsis, which is often several inches in length ; or the 
saccate cells of Valonia, sometimes as large as a walnut, and often as 
large as a hazel-nut ; or still more wonderful, the frond of Gaulerpa , 
sometimes 2 feet in length, and yet composed of a single cell ! The 
mere repetition of the vegetative parts, until they attain great 
dimensions, is not so remarkable as these. However, among exist- 
ing Algae are to be found some of the greatest masses of vegetable 
organisms. In the Northern Pacific a species of the genus Nereo- 
cystis attains a length of 300 feet, and spreads out on the surface 
of the water a great mass of green fronds, supported on a large 
floating bladder 6 or 7 feet long. Yet notwithstanding its enor- 
mous size, it appears to be only of annual growth. Much larger 
than this is the Macrocystis pyrifera, which Dr. Hooker, on 
reasonable grounds, has estimated to attain a length of 700 feet. 
And, it may be as regards external form, we have in the arbo- 
rescent Lessonias a nearer approach to these Devonian Algae. 
These Algae attain a height of 25 or 30 feet, and their stems, 
from their exogenous growth, are sometimes a foot in diameter. 
The pseudo-exogenous growth is well known in different Algae. 
Reference has already been made to the giant Lessonias of Antarctic 
seas, but I might adduce the Laminarias of our own shores in 
which this exogenous growth also occurs. In mentioning these two 
genera, both belonging to the same group (Laminar iaceas) of the 
Melanosperms, and both characterized by the possession of pseudo- 
exogenous stems, it is worth adding, by way of caution in generalizing, 
that the method of growth is different in the two genera. In 
