§ 279. Many commonplace facts indicate that the mechanical strains to which upright-growing plants are exposed, themselves cause increase of the dense deposits by which such plants are enabled to resist such strains. There is the fact that the massiveness of a tree-trunk varies according to the stress habitually put upon it. If the contrast between the slender stem of a tree growing in a wood and the bulky stem of a kindred tree growing in the fields, be ascribed to difference of nutrition rather than difference of exposure to winds; there is still the fact that a tree trained against a wall has a less bulky stem than a tree of the same kind growing unsupported; and that between the long weak branches of the one and the stiff ones of the other there are decided contrasts. If it be objected that a tree so trained and branches so borne have relatively less foliage, and that therefore these unlikenesses also are due to unlikenesses of general nutrition, which may in part be true; there are still such cases as those of garden plants, which when held up by tying them to sticks have weaker stems than when they are unpropped, and sink down if their props are taken away. Again, there is the evidence supplied by roots. Though the contrast between the feeble roots of a sheltered tree and the strong roots of an exposed tree, may, like the contrast of their stems, be mainly due to difference of nutrition, and therefore supplies but doubtful evidence, we get tolerably clear evidence where trees growing on inclined rocky surfaces, send into crevices that afford little moisture or nutriment, roots which nevertheless become thick where they are so directed as to bear great strains. Suspicion thus raised is strengthened into conviction by special evidences occurring in the places where they are to be expected. The Cactuses, with their succulent growths that pass into woody growths slowly and irregularly, give us the opportunity of tracing the conditions under which the wood is formed. Good examples occur in the genus _Cereus_, and especially in forms like _C. crenulatus_. Here, from a massive vertically-growing rod of fleshy tissue, two inches or more in diameter, there grow at intervals lateral rods similarly bulky, which, quickly curving themselves, take vertical directions. One of these heavy branches puts great strains on its own substance and that of the stem at their point of junction; and here both of them become brown and hard, while they continue green and succulent all around. Such differentiations may be traced internally before they are visible on the surface. If a joint of an _Opuntia_ be sliced through longitudinally, the greater resistance to the knife all around the narrow neck, indicates there a larger deposit of lignin than elsewhere; and a section of the tissue placed under the microscope, exhibits at the narrowest part a concentration of the woody and vascular bundles. Clear evidence of another kind has been noted by Mr.