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anisms。
With these it is practicable to control the principal conditions of cultivation and to vary them in various ways。 By this means it has been demonstrated that each developmental stage depends upon special external conditions； and in cases where our knowledge is sufficient； a particular stage may be obtained at will。 In the Green Algae （See Klebs； 〃Die Bedingung der Fortpflanzung。。。〃； Jena； 1896； also 〃Jahrb。 fur Wiss。 Bot。〃 1898 and 1900； 〃Probleme der Entwickelung； III。〃 〃Biol。 Centralbl。〃 1904； page 452。）； as in the case of Fungi； we may classify the stages of development into purely vegetative growth （growth； cell…division； branching）； asexual reproduction （formation of zoospores； conidia） and sexual processes （formation of male and female sexual organs）。 By modifying the external conditions it is possible to induce algae or fungi （Vaucheria； Saprolegnia） to grow continuously for several years or； in the course of a few days； to die after an enormous production of asexual or sexual cells。 In some instances even an almost complete stoppage of growth may be caused； reproductive cells being scarcely formed before the organism is again compelled to resort to reproduction。 Thus the sequence of the different stages in development can be modified as we may desire。
The result of a more thorough investigation of the determining conditions appears to produce at first sight a confused impression of all sorts of possibilities。 Even closely allied species exhibit differences in regard to the connection between their development and external conditions。 It is especially noteworthy that the same form in development may be produced as the result of very different alterations in the environment。 At the same time we can undoubtedly detect a certain unity in the multiplicity of the individual phenomena。
If we compare the factors essential for the different stages in development； we see that the question always resolves itself into one of modification of similar conditions common to all life…processes。 We should rather have inferred that there exist specific external stimuli for each developmental stage； for instance； certain chemical agencies。 Experiments hitherto made support the conclusion that QUANTITATIVE alterations in the general conditions of life produce different types of development。 An alga or a fungus grows so long as all the conditions of nutrition remain at a certain optimum for growth。 In order to bring about asexual reproduction； e。g。 the formation of zoospores； it is sometimes necessary to increase the degree of intensity of external factors； sometimes； on the other hand； these must be reduced in intensity。 In the case of many algae a decrease in light…intensity or in the amount of salts in the culture solution； or in the temperature； induces asexual reproduction； while in others； on the contrary； an increase in regard to each of these factors is required to produce the same result。 This holds good for the quantitative variations which induce sexual reproduction in algae。 The controlling factor is found to be a reduction in the supply of nutritive salts and the exposure of the plants to prolonged illumination or； better still； an increase in the intensity of the light； the efficiency of illumination depending on the consequent formation of organic substances such as carbohydrates。
The quantitative alterations of external conditions may be spoken of as releasing stimuli。 They produce； in the complex equilibrium of the cell； quantitative modifications in the arrangement and distribution of mass； by means of which other chemical processes are at once set in motion； and finally a new condition of equilibrium is attained。 But the commonly expressed view that the environment can as a rule act only as a releasing agent is incorrect； because it overlooks an essential point。 The power of a cell to receive stimuli is only acquired as the result of previous nutrition； which has produced a definite condition of concentration of different substances。 Quantities are in this case the determining factors。 The distribution of quantities is especially important in the sexual reproduction of algae； for which a vigorous production of the materials formed during carbon…assimilation appears to be essential。
In the Flowering plants； on the other hand； for reasons already mentioned； the whole problem is more complicated。 Investigations on changes in the course of development of fertilised eggs have hitherto been unsuccessful； the difficulty of influencing egg…cells deeply immersed in tissue constitutes a serious obstacle。 Other parts of plants are； however； convenient objects of experiment； e。g。 the growing apices of buds which serve as cuttings for reproductive purposes； or buds on tubers； runners； rhizomes； etc。 A growing apex consists of cells capable of division in which； as in egg…cells； a complete series of latent possibilities of development is embodied。 Which of these possibilities becomes effective depends upon the action of the outer world transmitted by organs concerned with nutrition。
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