darwin and modern science-第59章
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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 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 wi
