§ 1. We have hitherto spoken of each shoot as either straight or only warped by its spiral tendency; but no shoot of any length, except those of the sapling, ever can be straight; for, as the family of leaves which it bears are forced unanimously to take some given direction in search of food or light, the stalk necessarily obeys the same impulse, and bends itself so as to sustain them in their adopted position, with the greatest ease to itself and comfort for them.

In doing this, it has two main influences to comply or contend with: the first, the direct action of the leaves in drawing it this way or that, as they themselves seek particular situations; the second, the pressure of their absolute weight after they have taken their places, depressing each bough in a given degree; the leverage increasing as the leaf extends. To these principal forces may frequently be added that of some prevalent wind, which, on a majority of days in the year, bends the bough, leaves and all, for hours together, out of its normal position. Owing to these three forces, the shoot is nearly sure to be curved in at least two directions; that is to say, not merely as the rim of a wine-glass is curved (so that, looking at it horizontally, the circle becomes a straight line), but as the edge of a lip or an eyebrow is curved, partly upwards, partly forwards, so that in no possible perspective can it be seen as a straight line.