The widely spread power of reacting to wounding affords a very valuable means of inducing a fresh development of buds and roots on places where they do not occur in normal circumstances. Injury creates special conditions, but little is known as yet in regard to alterations directly produced in this way. Where the injury consists in the separation of an organ from its normal connections, the factors concerned are more comprehensible. A detached leaf, e.g., is at once cut off from a supply of water and salts, and is deprived of the means of getting rid of organic substances which it produces; the result is a considerable alteration in the degree of concentration. No experimental investigation on these lines has yet been made. Our ignorance has often led to the view that we are dealing with a force whose specific quality is the restitution of the parts lost by operation; the proof, therefore, that in certain cases a similar production of new roots or buds may be induced without previous injury and simply by a change in external conditions assumes an importance. (Klebs, "Willkurliche Entwickelung", page 100; also, "Probleme der Entwickelung", "Biol. Centralbl." 1904, page 610.)A specially striking phenomenon of regeneration, exhibited also by uninjured plants, is afforded by polarity, which was discovered by Vochting. (See the classic work of Vochting, "Ueber Organbildung im Pflanzenreich", I. Bonn, 1888; also "Bot. Zeit. 1906, page 101; cf. Goebel, "Experimentelle Morphologie", Leipzig and Berlin, 1908, Section V, Polaritat.) It is found, for example, that roots are formed from the base of a detached piece of stem and shoots from the apex. Within the limits of this essay it is impossible to go into this difficult question; it is, however, important from the point of view of our general survey to emphasise the fact that the physiological distinctions between base and apex of pieces of stem are only of a quantitative kind, that is, they consist in the inhibition of certain phenomena or in favouring them. As a matter of fact roots may be produced from the apices of willows and cuttings of other plants; the distinction is thus obliterated under the influence of environment. The fixed polarity of cuttings from full grown stems cannot be destroyed; it is the expression of previous development.
Vochting speaks of polarity as a fixed inherited character. This is an unconvincing conclusion, as nothing can be deduced from our present knowledge as to the causes which led up to polarity. We know that the fertilised egg, like the embryo, is fixed at one end by which it hangs freely in the embryo-sac and afterwards in the endosperm. From the first, therefore, the two ends have different natures, and these are revealed in the differentiation into root-apex and stem-apex. A definite direction in the flow of food-substances is correlated with this arrangement, and this eventually leads to a polarity in the tissues. This view requires experimental proof, which in the case of the egg-cells of flowering plants hardly appears possible; but it derives considerable support from the fact that in herbaceous plants, e.g. Sempervivum (Klebs, "Variationen der Bluten", "Jahrb. Wiss. Bot." 1905, page 260.), rosettes or flower-shoots are formed in response to external conditions at the base, in the middle, or at the apex of the stem, so that polarity as it occurs under normal conditions cannot be the result of unalterable hereditary factors. On the other hand, the lower plants should furnish decisive evidence on this question, and the experiments of Stahl, Winkler, Kniep, and others indicate the right method of attacking the problem.
The relation of leaf-form to environment has often been investigated and is well known. The leaves of bog and water plants (Cf.Goebel, loc. cit. chapter II.; also Gluck, "Untersuchungen uber Wasser- und Sumpfgewachse", Jena, Vols. I.-II. 1905-06.) afford the most striking examples of modifications: according as they are grown in water, moist or dry air, the form of the species characteristic of the particular habitat is produced, since the stems are also modified. To the same group of phenomena belongs the modification of the forms of leaves and stems in plants on transplantation from the plains to the mountains (Bonnier, "Recherches sur l'Anatomie experimentale des Vegetaux", Corbeil, 1895.) or vice versa.
Such variations are by no means isolated examples. All plants exhibit a definite alteration in form as the result of prolonged cultivation in moist or dry air, in strong or feeble light, or in darkness, or in salt solutions of different composition and strength.
Every individual which is exposed to definite combinations of external factors exhibits eventually the same type of modification. This is the type of variation which Darwin termed "definite." It is easy to realise that indefinite or fluctuating variations belong essentially to the same class of phenomena; both are reactions to changes in environment. In the production of individual variations two different influences undoubtedly cooperate. One set of variations is caused by different external conditions, during the production, either of sexual cells or of vegetative primordia; another set is the result of varying external conditions during the development of the embryo into an ***** plant. The two sets of influences cannot as yet be sharply differentiated. If, for purposes of vegetative reproduction, we select pieces of the same parent-plant of a pure species, the second type of variation predominates. Individual fluctuations depend essentially in such cases on small variations in environment during development.