18 
THE PRINCIPLES OF BIOLOGY. 
Tlie aggregates of the third order are with very few ex¬ 
ceptions fixed, and even in those exceptions ( Lemna ) show 
distinct bilaterality, or dorsi-ventrality. In uniaxial plants, the 
bird’s eye type is radial symmetry, e.g., a Palm. Divergences, 
where met with, affect usually the flower only, e.g., Foxglove, 
Wild Hyacinth. In multiaxial plants the general type again 
is radial, but constantly broken, even in the open, by 
inequality of sun’s rays, and the prevalent direction ot the 
wind. The type of the root is more distinctly radial, in 
cases, indeed, we might almost describe it as hemispherical, 
but divergences arise from inequalities in the hardness of the 
soil, and in the distribution of moisture, and of internal food 
supply. 
The nearest approach to simple spherical symmetry in 
branches is in the “ buds” of the yeast plant, and the conidia 
of many fungi. The most of simpler plants have their 
branches radial in type, and often radial in development, and 
in these, as in all cases, the branch is either a repetition or 
a simplification of the axis, and the attached differs strongly 
from the free end. Floride®, Characeae, and Equisetacese 
furnish abundant illustrations. Of the larger plants, many 
do not branch at all; those which do produce their branches 
originally in radial wise, though by stress of circumstances 
they may ultimately become strongly bilateral, e.g., the Lime 
and the Fir ; the two main agents in this change are light and 
gravity. The influence of the action of light is very strongly 
manifest in such a case as the Ivy ; where growing out free 
above a small tree which it has enveloped, it seems hardly 
the same plant as when clinging to the face of a wall. 
In leaves, highly differentiated as they are, spherical 
symmetry is entirely wanting, and even radial (peltate) 
symmetry is rare. This latter is met with in certain cases, 
such as Oxahs and Marsilia, amongst compound leaves, and 
Nelumbium, Troyceohim, Victoria regia, and Hydrocotyle, 
amongst simple leaves. In all these cases we have either 
vertical leaf-stalks, from a more or less creeping stem, or else 
leaf-stalks so long that they can carry out the laminae to a 
considerable distance from the centre. Interesting transitions 
are furnished by such a plant as the Lupine, in which the 
radical leaves show proximate radial symmetry, but with the 
external leaflets longest while the cauline (stem) leaves 
successively become more strongly bilateral. The influence 
of light is here probably the most important factor, each leaf, 
or leaflet, tending to grow uncovered by the shade of its next 
successor. An illustration of this is provided by two semi¬ 
aquatics of generally like habit; in the Water-cress, with 
