degree of its nutrition. Every fern-leaf exemplifies the same general truth still better. Whether each sub-frond remains an undeveloped wing of the main frond, or whether it organizes itself into a group of frondlets borne by a secondary rib, or whether, going further, as it often does, it gives rise to tertiary ribs bearing frondlets, is determined by the supply of materials for growth; since such higher developments are most marked at points where the nutrition is greatest; namely, next the stem. But the clearest evidence is afforded among the _Algæ_, which, not drawing nutriment from roots, have their parts much less mutually dependent; and are therefore capable of showing more clearly, how any part may remain an appendage or may become the parent of appendages, according to circumstances. In the annexed Fig. 130, representing a branch of _Ptilota plumosa_, we see how a wing grows into a wing-bearing branch if its nutrition passes a certain point. This form, so strikingly like that of the feathery crystallizations of many inorganic substances, implies that, as in such crystallizations, the simplicity or complexity of structure at any place depends on the quantity of matter that has to be arranged at that place in a given time.[14] [Illustration: Fig. 130.] Hence, then, we are not without an interpretation of those over-developments which the phænogamic axis occasionally undergoes. Fig. 104, represents the phænogamic bud in its rudimentary state. The lateral process _b_, which ordinarily becomes a foliar appendage, differs very little from the terminal process _c_, which is to become an axis--differs mainly in having, at this period when its form is being determined, a smaller bulk. If while thus undifferentiated, its nutrition remains inferior to that of the terminal process, it becomes moulded into a part that is subordinate to the general axis. But if, as sometimes happens, there is supplied to it such an abundance of the materials needful for growth, that it becomes as large as the terminal process; then we may naturally expect it to begin moulding itself round an axis of its own: a foliar organ will be replaced by an axial organ. And this result will be especially liable to occur, when the growth of the axis has been previously undergoing that arrest which leads to the formation of a flower; that is when, from defect of materials, the terminal process has almost ceased to increase, and when some concurrence of favourable causes brings a sudden access of sap which reaches the lateral processes before it reaches the terminal process.[15]