The taproot of a tree, as we have seen,[86] grows straight downwards towards the centre of the earth in obedience to the stimulus given by the pull of gravitation. The same stimulus impels the stem to shoot upwards, and the other roots and the branches to grow more or less laterally. New growth always takes place at the extreme tip of the shoot or root. Lay bare the taproot, cut away this growing tip, and that root can grow no more; no fresh tip charged with vegetative vitality can form itself over the scar. But mark what happens! The nearest lateral root, instead of pursuing its normal course, straightway begins to bend downwards and takes the place of the mutilated taproot. Similarly if the leading shoot of the stem is nipped off, the nearest lateral branch will turn upwards. In this case the lateral root or shoot has not been subjected to any new influences whatever, or at least to none of a chemical or physical nature. Yet it responds, not to anything affecting itself, but to the needs of the organism as a whole.[87] None of the forces which living organisms have in common with minerals will account for this kind of response. How are we to represent to our minds the nature of the forces which apply to the innumerable cases of which the above is a type? Reinke, who deals exhaustively with this question, conceives the vitality of living things, manifested in growth, development, and reproduction, as lodged in what he calls “Dominants.”[88] These dominants exist in all parts of the organism, and govern those processes which ordinary physical laws do not explain, _i.e._ the phenomena which are specifically vital. They are not themselves chemical or physical energies, but they guide these energies toward the fulfilment of the objects of life. “Dominants,” he writes,[89] “are those secondary[90] forces in the organism whose existence we recognize in their operations, but which we cannot further analyse. Thus I understand under this form that principle of control which takes effect in every organism and which sways whatever energies are available just as men use tools and machines. Since this control is manifold in its manifestations, one is obliged, when seeking for a technical designation for it, to express it in the plural. The dominants are therefore an abstraction; a symbol for phenomena, just like the conceptions, Force, Matter, the Atom, etc.; the term has been devised in order to provide a short explanatory description of certain essential processes. “I therefore repel the objection, if anyone should make it, that the dominants are a fiction, a troop of ghosts with which I have peopled the cells and organs of animals and plants. They are, in some sense, merely a paraphrase of the description of certain phenomena, a personification of forces not to be ranged under the conception of energy—the _directive_ impulses in the animal and vegetable world.”