Reduced to their most general expression, the truths above shadowed forth are these:--That group of morphological units, or cells, which we see integrated into the compound unit called a leaf, has, in each higher plant, a typical form, due to the special arrangement of these cells around a mid-rib and veins. If the multiplication of morphological units, at the time when the leaf-bud is taking on its main outlines, exceeds a certain limit, these units begin to arrange themselves round secondary centres, or lines of growth, in such ways as to repeat, in part or wholly, the typical form: the larger veins become transformed into imperfect mid-ribs of partially independent leaves; or into complete mid-ribs of quite separate leaves. And as there goes on this transition from a single aggregate of cells to a group of such aggregates, there simultaneously arises, by similarly insensible steps, a distinct structure which supports the several aggregates thus produced, and unites them into a compound aggregate. These phenomena should be carefully studied; since they give us a key to more involved phenomena.[6] § 189. Thus far we have dealt with leaves ordinarily so-called: briefly indicating the homologies between the parts of the simple and the compound. Let us now turn to the homologies among foliar organs in general. These have been made familiar to readers of natural history by popularized outlines of _The Metamorphosis of Plants_--a title, by the way, which is far too extensive; since the phenomena treated of under it, form but a small portion of those it properly includes.