278 
THE PRINCIPLES OF BIOLOGY. 
THE PRINCIPLES OF BIOLOGY. 
BY HERBERT SPENCER. 
Exposition of Part III., Chapter 3. 
BY W. B. GROVE, B.A. 
Tliis chapter deals with the extension of the theory 
elaborated in the two previous chapters, to the Phanerogams, 
but it is so complicated and difficult to understand without 
diagrams that it will be advisable merely to state the con¬ 
clusions arrived at, without attempting to show the reasons 
on which they are founded. 
Mr. Spencer first shows that by observing the various 
forms of leaves of a Bramble stem, from the simplest to the 
most compound, we may have suggested to us the idea that 
the mid-rib of a leaf can assume the function of an axis ; a 
phyllome becomes a caulome. The same thing is illustrated 
by Eucalyptus, Acacia, and Ruscus. 
The chief problem to be solved is to show how the frond 
of an Acrogen can be transformed into a true caulome. 
After illustrating the change of leaf into stem by examples 
taken from the Euphorbiacese and Cactaceae, it is pointed out 
that the first beginning of the process lies in that repeated 
proliferation of the frond of which examples may be found in 
the Jiuigermanniae. By this means, a frond which was at 
first procumbent on the ground can take a step, literally, into 
a higher life. But before this process can be carried to any 
great extent, it will be necessary that the frond should be 
strengthened. This can take place in two ways, and, 
apparently, in two ways only:—(1) by an infolding of the 
margins of the frond, on the principle of a hollow iron 
column ; this necessarily gives rise to the mode of growth 
called Endogenous ; (2) by a thickening of the mid-rib ; 
this produces that type of structure which is called Exogenous. 
From this it follows that the unit of composition of a 
Flowering Plant is that portion of a stem which answers 
to the original frond ; i.e., an internode with its superjacent 
leaf. The proliferation of this gives rise to the internode 
above with its leaf. But occasionally a frond can give off a 
double proliferation. In this lies the origin of an axillary 
bud, when present. The axillary bud, therefore, is not 
essential to the leaf, contrary to the doctrine of those 
German theorists who would stifle all morphology within the 
iron folds of their artificial types. 
