按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
osen〃； 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 kn