20 
THE PRINCIPLES OF BIOLOGY. 
flowers, diverge widely from the radial type. Interesting 
examples of the kind of agency which may effect change are 
provided by the Candy tuft, the wild Guelder rose, and 
several Umbellifers, as well as by a large group of the 
Composite. In all these cases the flowers form a more or 
less flattened group, and while the central flowers are radial 
(as far as general contour goes), the external flowers are 
entirely bilateral, extending largely on their external side. 
Clearly here, as in the inverted flowers of Orchids, and the 
pendant raceme of the Laburnum, mechanical conditions, 
though of a different class, come into play. Again, the 
under half of a bilateral flower tends to project beyond the 
upper half. Whence is this ? As Mr. Spencer points out, 
these parts contain no chlorophyll, and hence, he assumes— 
“ the tendency to grow most where the supply of light is 
greatest is less decided, if not absent.” But perhaps he has 
told here but half of the tale. The tendency of green parts 
to grow largest in the greatest light is strongly marked; but 
it is equally true that light has certain mechanical effects 
which are disadvantageous to growth, and that, therefore, in a 
part whose growth is independent of light, the growth, 
cceteris paribus, will he greatest where the light is least. 
Nor do I think the mechanical reasons referred to above must 
have undue weight attached to them. The most marked cases 
of bilaterality in flowers are those where insect agency is the 
medium for fertilisation, and the progressive influence of 
this agency upon the flower introduces a disturbing factor 
of the first class. 
Another point of prime interest in an enquiry such as this 
can he only touched upon here, and it is this : that while in 
the flower as a whole the radial type greatly outnumbers the 
bilateral, in the ovary the bilateral type hugely preponderates. 
Excluding Leguminosae and EupliorbiaceaB there is no natural 
order of primary numeideal importance which has not a 
bicarpellary ovary. 
As to the question of the shapes of vegetal cells, discussed 
by Mr. Spencer with great brevity in Chapter XI., space will 
not permit us to do more than indicate a few points of 
importance. Mr. Spencer’s own conceptions are remarkably 
circumscribed. A point we should like to lay stress upon is 
the different shapes of cells in the interior tissues of the leaf. 
Taking a typical flatly exposed leaf, under its upper surface 
are one or more layers of cells whose long axis is vertical to 
the surface, and in the same leaf the number of these layers 
will vary with the exposure of the leaf. The more light, the 
more the layers. In plants of shady places, and in shaded 
