We commence as before with plants. We have to consider, first, the several kinds of modification in shape they have undergone; and, second, the relations between these kinds of modification and their factors. Let us glance at the leading questions that have to be answered. § 213. Irrespective of their degrees of composition, plants may, and do, become changed in their general forms. Are their changes capable of being formulated? The inquiry which meets us at the outset is--does a plant’s shape admit of being expressed in any universal terms?--terms that remain the same for all genera, orders, and classes. After plants considered as wholes, have to be considered their proximate components, which vary with their degrees of composition, and in the highest plants are what we call branches. Is there any law traceable among the contrasted shapes of different branches in the same plant? Do the relative developments of parts in the same branch conform to any law? And are these laws, if they exist, allied with one another and with that to which the shape of the whole plant conforms? Descending to the components of these components, which in developed plants we distinguish as leaves, there meet us kindred questions respecting their relative sizes, their relative shapes, and their shapes as compared with those of foliar organs in general. Of their morphological differentiations, also, it has to be asked whether they exemplify any truth that is exemplified by the entire plant and by its larger parts. Then, a step lower, we come down to those morphological units of which leaves and fronds consist; and concerning these arise parallel inquiries touching their divergences from one another and from cells in general. The problems thus put together in several groups cannot of course be rigorously separated. Evolution presupposes transitions which make all such classings more or less conventional; and adherence to them must be subordinate to the needs of the occasion. § 214. In studying the causes of the morphological differentiations thus divided out and prospectively generalized, we shall have to bear in mind several orders of forces which it will be well briefly to specify. Growth tends inevitably to initiate changes in the shape of any aggregate, by altering both the amounts of the incident forces and the forces which the parts exert on one another. With the mechanical actions this is obvious. Matter that is sensibly plastic cannot be increased in mass without undergoing a change in its proportions, consequent on the diminished ratio of its cohesive force to the force of gravitation. With the physiological actions it is equally obvious. Increase of size, other things equal, alters the relations of the parts to the material and dynamical factors of nutrition; and by so affecting differently the nutrition of different parts, initiates further changes of proportions.