darwin and modern science-第60章
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Of the different stages which a flowering plant passes through in the course of its development we will deal only with one in order to show that； in spite of its great complexity； the problem is； in essentials； equally open to attack in the higher plants and in the simplest organisms。 The most important stage in the life of a flowering plant is the transition from purely vegetative growth to sexual reproductionthat is； the production of flowers。 In certain cases it can be demonstrated that there is no internal cause； dependent simply on the specific structure； which compels a plant to produce its flowers after a definite period of vegetative growth。 （Klebs； 〃Willkurliche Entwickelungsanderungen〃； Jena 1903； see also 〃Probleme der Entwickelung〃； I。 II。 〃Centralbl。〃 1904。）
One extreme case； that of exceptionally early flowering； has been observed in nature and more often in cultivation。 A number of plants under certain conditions are able to flower soon after germination。 （Cf。 numerous records of this kind by Diels； 〃Jugendformen und Bluten〃； Berlin； 1906。） This shortening of the period of development is exhibited in the most striking form in trees； as in the oak （Mobius； 〃Beitrage zur Lehre von der Fortpflanzung〃； Jena； 1897； page 89。）； flowering seedlings of which have been observed from one to three years old； whereas normally the tree does not flower until it is sixty or eighty years old。
Another extreme case is represented by prolonged vegetative growth leading to the complete suppression of flower…production。 This result may be obtained with several plants； such as Glechoma； the sugar beet； Digitalis； and others； if they are kept during the winter in a warm； damp atmosphere； and in rich soil； in the following spring or summer they fail to flower。 （Klebs； 〃Willkurliche Aenderungen〃； etc。 Jena； 1903； page 130。） Theoretically； however； experiments are of greater importance in which the production of flowers is inhibited by very favourable conditions of nutrition （Klebs； 〃Ueber kunstliche Metamorphosen〃； Stuttgart； 1906； page 115 （〃Abh。 Naturf。 Ges。 Halle〃； XXV。） occurring at the normal flowering period。 Even in the case of plants of Sempervivum several years old； which； as is shown by control experiments on precisely similar plants； are on the point of flowering； flowering is rendered impossible if they are forced to very vigorous growth by an abundant supply of water and salts in the spring。 Flowering； however； occurs； if such plants are cultivated in relatively dry sandy soil and in the presence of strong light。 Careful researches into the conditions of growth have led； in the cases Sempervivum； to the following results： （1） With a strong light and vigorous carbon…assimilation a considerably increased supply of water and nutritive salts produces active vegetative growth。 （2） With a vigorous carbon…assimilation in strong light； and a decrease in the supply of water and salts active flower…production is induced。 （3） If an average supply of water and salts is given both processes are possible； the intensity of carbon…assimilation determines which of the two is manifested。 A diminution in the production of organic substances； particularly of carbohydrates； induces vegetative growth。 This can be effected by culture in feeble light or in light deprived of the yellow…red rays： on the other hand； flower…production follows an increase in light…intensity。 These results are essentially in agreement with well…known observations on cultivated plants； according to which； the application of much moisture； after a plentiful supply of manure composed of inorganic salts； hinders the flower…production of many vegetables； while a decrease in the supply of water and salts favours flowering。
ii。 INFLUENCE OF THE ENVIRONMENT ON THE FORM OF SINGLE ORGANS。 （A considerable number of observations bearing on this question are given by Goebel in his 〃Experimentelle Morphologie der Pflanzen〃； Leipzig； 1908。 It is not possible to deal here with the alteration in anatomical structure； cf。 Kuster； 〃Pathologische Pflanzenanatomie〃； Jena； 1903。）
If we look closely into the development of a flowering plant； we notice that in a given species differently formed organs occur in definite positions。 In a potato plant colourless runners are formed from the base of the main stem which grow underground and produce tubers at their tips： from a higher level foliage shoots arise nearer the apex。 External appearances suggest that both the place of origin and the form of these organs were predetermined in the egg…cell or in the tuber。 But it was shown experimentally by the well…known investigator Knight （Knight； 〃Selection from the Physiological and Horticultural Papers〃； London； 1841。） that tubers may be developed on the aerial stem in place of foliage shoots。 These observations were considerably extended by Vochting。 （Vochting； 〃Ueber die Bildung der Knollen〃； Cassel； 1887； see also 〃Bot。 Zeit。〃 1902； 87。） In one kind of potato； germinating tubers were induced to form foliage shoots under the influence of a higher temperature； at a lower temperature they formed tuber…bearing shoots。 Many other examples of the conversion of foliage…shoots into runners and rhizomes； or vice versa； have been described by Goebel and others。 As in the asexual reproduction of algae quantitative alteration in the amount of moisture； light； temperature； etc。 determines whether this or that form of shoot is produced。 If the primordia of these organs are exposed to altered conditions of nutrition at a sufficiently early stage a complete substitution of one organ for another is effected。 If the rudiment has reached a certain stage in development before it is exposed to these influences； extraordinary intermediate forms are obtained； bearing the characters of both organs。
The study of regeneration following injury is of greater importance as regards the problem of the development and place of origin of organs。 （Reference may be made to the full summary of results given by Goebel in his 〃Experimentelle Morphologie〃； Leipzig and Berlin； 1908； Section IV。） Only in relatively very rare cases is there a complete re…formation of the injured organ itself； as e。g。 in the growing…apex。 Much more commonly injury leads to the development of complementary formations； it may be the rejuvenescence of a hitherto dormant rudiment； or it may be the formation of such ab initio。 In all organs； stems； roots； leaves； as well as inflorescences； this kind of regeneration； which occurs in a great variety of ways according to the species； may be observed on detached pieces of the plant。 Cases are also known； such； for example； as the leaves of many plants which readily form roots but not shoots； where a complete regeneration does not occur。
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
