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velopment。 They also possess special interest in relation to the question of heredity； a subject which does not at present concern us； as such deviations from normal development undoubtedly arise as individual variations induced by the influence of environment。
Abnormal developments of all kinds in stems； leaves； and flowers； may be produced by parasites； insects； or fungi。 They may also be induced by injury； as Blaringhem （Blaringhem； 〃Mutation et traumatismes〃； Paris； 1907。） has more particularly demonstrated； which； by cutting away the leading shoots of branches in an early stage of development； caused fasciation； torsion； anomalous flowers； etc。 The experiments of Blaringhem point to the probability that disturbances in the conditions of food…supply consequent on injury are the cause of the production of monstrosities。 This is certainly the case in my experiments with species of Sempervivum （Klebs； 〃Kunstliche Metamorphosen〃； Stuttgart； 1906。）； individuals； which at first formed normal flowers； produced a great variety of abnormalities as the result of changes in nutrition； we may call to mind the fact that the formation of inflorescences occurs normally when a vigorous production of organic compounds； such as starch； sugar； etc。 follows a diminution in the supply of mineral salts。 On the other hand； the development of inflorescences is entirely suppressed if； at a suitable moment before the actual foundations have been laid； water and mineral salts are supplied to the roots。 If； during the week when the inflorescence has just been laid down and is growing very slowly； the supply of water and salts is increased； the internal conditions of the cells are essentially changed。 At a later stage； after the elongation of the inflorescence； rosettes of leaves are produced instead of flowers； and structures intermediate between the two kinds of organs； a number of peculiar plant…forms are thus obtained （Cf。 Lotsy； 〃Vorlesungen uber Deszendenztheorien〃； Vol。 II。 pl。 3； Jena； 1908。） Abnormalities in the greatest variety are produced in flowers by varying the time at which the stimulus is applied； and by the cooperation of other factors such as temperature； darkness； etc。 In number and arrangement the several floral members vary within wide limits； sepals； petals； stamens； and carpels are altered in form and colour； a transformation of stamens to carpels and from carpels to stamens occurs in varying degrees。 The majority of the deviations observed had not previously been seen either under natural conditions or in cultivation； they were first brought to light through the influence of external factors。
Such transformations of flowers become apparent at a time； which is separated by about two months from the period at which the particular cause began to act。 There is； therefore； no close connection between the appearance of the modifications and the external conditions which prevail at the moment。 When we are ignorant of the causes which are operative so long before the results are seen； we gain the impression that such variations as occur are spontaneous or autonomous expressions of the inner nature of the plant。 It is much more likely that； as in Sempervivum； they were originally produced by an external stimulus which had previously reached the sexual cells or the young embryo。 In any case abnormalities of this kind appear to be of a special type as compared with ordinary fluctuating variations。 Darwin pointed out this difference； Bateson （Bateson； 〃Materials for the study of Variation〃； London； 1894； page 5。） has attempted to make the distinction sharper； at the same time emphasising its importance in heredity。
Bateson applies the term CONTINUOUS to small variations connected with one another by transitional stages； while those which are more striking and characterised from the first by a certain completeness； he names DISCONTINUOUS。 He drew attention to a great difficulty which stands in the way of Lamarck's hypothesis； as also of Darwin's view。 〃According to both theories； specific diversity of form is consequent upon diversity of environment； and diversity of environment is thus the ultimate measure of diversity of specific form。 Here then we meet the difficulty that diverse environments often shade into each other insensibly and form a continuous series； whereas the Specific Forms of life which are subject to them on the whole form a Discontinuous Series。〃 This difficulty is； however； not of fundamental importance as well authenticated facts have been adduced showing that by alteration of the environment discontinuous variations； such as alterations in the number and form of members of a flower； may be produced。 We can as yet no more explain how this happens than we can explain the existence of continuous variations。 We can only assert that both kinds of variation arise in response to quantitative alterations in external conditions。 The question as to which kind of variation is produced depends on the greater or less degree of alteration； it is correlated with the state of the particular cells at the moment。
In this short sketch it is only possible to deal superficially with a small part of the subject。 It has been clearly shown that in view of the general dependence of development on the factors of the environment a number of problems are ready for experimental treatment。 One must； however； not forget that the science of the physiology of form has not progressed beyond its initial stages。 Just now our first duty is to demonstrate the dependence on external factors in as many forms of plants as possible； in order to obtain a more thorough control of all the different plant…forms。 The problem is not only to produce at will （and independently of their normal mode of life） forms which occur in nature； but also to stimulate into operation potentialities which necessarily lie dormant under the conditions which prevail in nature。 The constitution of a species is much richer in possibilities of development than would appear to be the case under normal conditions。 It remains for man to stimulate into activity all the potentialities。
But the control of plant…form is only a preliminary stepthe foundation stones on which to erect a coherent scientific structure。 We must discover what are the internal processes in the cell produced by external factors； which as a necessary consequence result in the appearance of a definite form。 We are here brought into contact with the most obscure problem of life。 Progress can only be made pari passu with progress in physics and chemistry； and with the growth of our knowledge of nutrition； growth； etc。
Let us take one of the simplest casesan alteration in form。 A cylindrical cell of the alga Stigeoclonium assumes； as Livingstone （Livingstone； 〃On the nature of the stimulus which causes the change of form； etc。〃 〃Botanical Gazette〃； XXX。 1900； also XXXII。 1901。） has shown； a spherical form when the osmotic pressure of the culture fluid is increased； or a spore of Mucor； which； in a sugar solution grows into a branched filament； in the presence of a small quantity of acid （hydrogen ions） becomes a comparatively large sphere。 （Ritter； 〃Ueber Kugelhefe； etc。〃 〃Ber