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nce。 This conception of specific structure is specially important in its bearing on a further treatment of the subject。 In the original cell； eventually also in every cell of a plant； the characters which afterwards become apparent must exist somewhere； they are integral parts of the capabilities or potentialities of specific structure。 Thus not only the characters which are exhibited under ordinary conditions in nature； but also many others which become apparent only under special conditions （In this connection I leave out of account； as before； the idea of material carriers of heredity which since the publication of Darwin's Pangenesis hypothesis has been frequently suggested。 See my remarks in 〃Variationen der Bluten〃； 〃Pringsheim's Jahrb。 Wiss。 Bot。〃 1905； page 298； also Detto； 〃Biol。 Centralbl。〃 1907； page 81； 〃Die Erklarbarkeit der Ontogenese durch materielle Anlagen〃。）； are to be included as such potentialities in cells； the conception of specific structure includes the WHOLE OF THE POTENTIALITIES OF A SPECIES； specific structure comprises that which we must always assume without being able to explain it。
A relatively simple substance； such as oxalate of lime； is known under a great number of different crystalline forms belonging to different systems （Compare Kohl's work on 〃Anatomisch…phys。 Untersuchungen uber Kalksalze〃； etc。 Marburg； 1889。）； these may occur as single crystals； concretions or as concentric sphaerites。 The power to assume this variety of form is in some way inherent in the molecular structure； though we cannot； even in this case； explain the necessary connection between structure and crystalline form。 These potentialities can only become operative under the influence of external conditions； their stimulation into activity depends on the degree of concentration of the various solutions； on the nature of the particular calcium salt； on the acid or alkaline reactions。 Broadly speaking； the plant cell behaves in a similar way。 The manifestation of each form； which is inherent as a potentiality in the specific structure； is ultimately to be referred to external conditions。
An insight into this connection is； however； rendered exceedingly difficult； often quite impossible； because the environment never directly calls into action the potentialities。 Its influence is exerted on what we may call the inner world of the organism； the importance of which increases with the degree of differentiation。 The production of form in every plant depends upon processes in the interior of the cells； and the nature of these determines which among the possible characters is to be brought to light。 In no single case are we acquainted with the internal process responsible for the production of a particular form。 All possible factors may play a part； such as osmotic pressure； permeability of the protoplasm； the degree of concentration of the various chemical substances； etc。； all these factors should be included in the category of INTERNAL CONDITIONS。 This inner world appears the more hidden from our ken because it is always represented by a certain definite state； whether we are dealing with a single cell or with a small group of cells。 These have been produced from pre…existing cells and they in turn from others； the problem is constantly pushed back through a succession of generations until it becomes identified with that of the origin of species。
A way； however； is opened for investigation； experience teaches us that this inner world is not a constant factor： on the contrary； it appears to be very variable。 The dependence of VARIABLE INTERNAL on VARIABLE EXTERNAL conditions gives us the key with which research may open the door。 In the lower plants this dependence is at once apparent； each cell is directly subject to external influences。 In the higher plants with their different organs； these influences were transmitted to cells in course of development along exceedingly complex lines。 In the case of the growing…point of a bud； which is capable of producing a complete plant； direct influences play a much less important part than those exerted through other organs； particularly through the roots and leaves； which are essential in nutrition。 These correlations； as we may call them； are of the greatest importance as aids to an understanding of form…production。 When a bud is produced on a particular part of a plant； it undergoes definite internal modifications induced by the influence of other organs； the activity of which is governed by the environment； and as the result of this it develops along a certain direction； it may； for example； become a flower。 The particular direction of development is determined before the rudiment is differentiated and is exerted so strongly that further development ensues without interruption； even though the external conditions vary considerably and exert a positively inimical influence： this produces the impression that development proceeds entirely independently of the outer world。 The widespread belief that such independence exists is very premature and at all events unproven。
The state of the young rudiment is the outcome of previous influences of the external world communicated through other organs。 Experiments show that in certain cases； if the efficiency of roots and leaves as organs concerned with nutrition is interfered with； the production of flowers is affected； and their characters； which are normally very constant； undergo far…reaching modifications。 To find the right moment at which to make the necessary alteration in the environment is indeed difficult and in many cases not yet possible。 This is especially the case with fertilised eggs； which in a higher degree than buds have acquired； through parental influences； an apparently fixed internal organisation； and this seems to have pre…determined their development。 It is； however； highly probable that it will be possible； by influencing the parents； to alter the internal organisation and to switch off development on to other lines。
Having made these general observations I will now cite a few of the many facts at our disposal； in order to illustrate the methods and aim of the experimental methods of research。 As a matter of convenience I will deal separately with modification of development and with modification of single organs。
I。 EFFECT OF ENVIRONMENT UPON THE COURSE OF DEVELOPMENT。
Every plant； whether an alga or a flowering plant passes； under natural conditions； through a series of developmental stages characteristic of each species； and these consist in a regular sequence of definite forms。 It is impossible to form an opinion from mere observation and description as to what inner changes are essential for the production of the several forms。 We must endeavour to influence the inner factors by known external conditions in such a way that the individual stages in development are separately controlled and the order of their sequence determined at will by experimental treatment。 Such control over the course of development may be gained with special certainty in the case of the lower organisms。
With these it is practicable to control the principal conditions of cultivation and to va