178 BOTANY paut i 



2. The study of development or the physiology of form, changes 



of shape, and the mechanism of development. 



3. The study of movement. 



The full vital activity of the plant is only attained when a number 

 of conditions, which may be divided into internal and external, are 

 fulfilled. (^) The internal causes of life are connected with the 

 protoplasm. Its structure and organisation not only determine that 

 the changes which take place in the organism have a Adtal character, 

 but that the organism shows specific differences depending on the 

 descent of its protoplasm. These internal factors are only effective, 

 however, by continual interaction with the external world. The 

 environment not only provides the materials from which the body 

 of the plant is built up, but supplies energy in the form of the 

 vibrations of heat and light ; this energy is again used in numerous 

 processes in the life of the organism. The infiuences of the external 

 world also act, as has been already pointed out, as stimuli to which the 

 protoplasm of the plant continually reacts in the manifestations of its 

 life. These external influences can only be of use Avithin definite and 

 strictly limited ranges of intensity. The capacity of life of vegetable 

 protoplasm is only maintained within a relatively narrow range of 

 temperature ; full vital activity only occurs within still narrower 

 limits. An excess of light is injurious to life, as is too little warmth, 

 and it only requires a minute quantity of any poisonous substance to 

 destroy beyond recovery the specific structure of the protoplasm. 



The lower limit for the operation of an external factor is known as 

 the MINIMUM, the upper limit as the maximum, and the particular 

 grade of intensity of the factor at which any particular vital phenomenon 

 attains its highest expression is known as the optimum. Minimum, 

 optimum, and maximum, but especially the optimum, are by no means 

 fixed points for the organism. They are changeable Avithin certain 

 limits, on the one hand, Avith the duration of action of the factor ; on 

 the other, in relation to other conditions of life. These so-called 

 " cardinal points " are usually different for the various vital processes 

 of the same plant. 



The dependence of the vital plienomena on external factors can be graphically 

 represented and made clearer by means of curves. If the intensity of the factor 

 (degrees of temjjerature, intensity of light) is marked on the abscissa, and the 

 intensity of the reaction of the i)lant on the ordinate, curves of the form AE (Fig. 

 175), which are known as optimum curves, are usually obtained. The summit of 

 this curve corresponds to the optimum. 



Curves, in which the dependence of chemical reactions, on for instance tempera- 

 ture, are represented, usually exhibit another shape (AB) : they are for the most part 

 logarithmic curves with no indication of an optimum. This does not express any 

 fundamental dilference between the reactions within the organisin and inorganic 

 reactions. Probably optimum curves are always the resultants of two kinds of 

 elfect of the same factor, one accelerating, the other destructive. Thus, for instance, 



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