immense difference between a self-conscious, self-guiding organism, and a dead machine requiring to be supplied and regulated by an external consciousness; yet in the fundamental physical necessity for energetic material, either as food or as fuel, both mechanisms follow essentially the self-same mechanical laws.
But what has all this to do with the origin of fruits? Very little at first sight, indeed, yet everything when we look at the bottom of the question. In fact, what is thus true of animals and steam-engines is equally true of plants. No motion can take place in a growing shoot without the aid of solar energy, directly supplied by the sunshine, or indirectly laid by in the older tissues. In the green parts of a plant this energy is immediately derived from the bounteous light which bathes and vivifies the leaves on every side; but in many other portions of the vegetable organism, energies previously accumulated by older organs are perpetually being utilized, for the production of movement and growth, by lazy structures which cannot work for themselves, and so feed upon the useful materials collected for them by more industrious members of the plant-commonwealth. Especially is this the case with those expensive organs which are concerned in perpetuating the species to future generations. A flower or a seed cannot directly transform waves of light into chemical separation of atoms; they depend for their growth and the due performance of their important functions upon similar separations already carried on for their behoof by the green leaves on whose bounty they rely for proper subsistence. Carbon, set free from oxygen in the leaves, has been carried to them in loose combinations by the sap; and as the bud unfolds or the seed germinates, the oxygen once more unites with this carbon (just as it unites in the furnace of the steam-engine, or the recesses of the animal body), and motion is thereby rendered possible. But without such an access of free oxygen to recombine with the energetic materials, the blossom or the embryo could never grow at all. So we may regard these portions of a plant, incapable of self-support, and dependent for their due function upon energetic compounds laid by elsewhere, as the exact analogues of the animal or the steam-engine. They are in fact similar mechanisms, where food is being used up, and fuel is being consumed; and we find accordingly, as we might naturally expect, not only that motion results, but also that heat is evolved in quantities quite sufficient to be measured by very delicate thermometers.
Now, every growing portion of a plant shares, more or less, in this animal function of feeding upon previously-fabricated nutriment. But there are two sets of organs, both intended ultimately to subserve the same purpose, in which that function becomes especially apparent. The first is in the case of the whole regular reproductive mechanism, including in that term buds, flowers, fruits, and seeds; the second is in the case of such subsidiary reproductive devices as tubers, rhizomes, corms, and all the other varieties of underground stems or roots, which