428 TRANSFORMATION OF ENERGY 



quently either affect different parts of the plants with different intensity or the 

 intensity itself alters from time to time. Every stimulus, as we have already 

 sufficiently shown, is a releasing stimulus only. That the formal conditions 

 (general vital conditions) operate also as stimuli has not always been clearly 

 appreciated ; but in the majority of cases it is impossible to regard them in any 

 other light, and it is often, indeed, extremely difficult to differentiate them from 

 special stimuli. 



Given the general conditions the special stimuli lead to a number of internal 

 processes which we shall get to know about more exactly later on, and, finally, 

 to movement ; this we may term the stimulus reaction, or, more accurately, the 

 visible final reaction. We say that the stimulus induces a certain movement ; 

 the movement is therefore known as an induced or paratonic movement. Move- 

 ments, which are outwardly indistinguishable from paratonic movements, are 

 also frequently to be met with, which are not so induced ; these we speak of as 

 autonomous movements. 



We must now attempt to formulate some suitable classification of stimulus 

 movements. We might group them either according to the nature of the stimulus, 

 that is to say, movements induced by heat, light, and so on, or we may base it 

 on the nature of the reaction, or, finally, on the biological significance of the move- 

 ment. We will select the nature of the reaction as the principle to follow, and 

 distinguish first of all the reactions exhibited by motile organisms, which we 

 shall discuss in the two final lectures, as opposed to the reactions manifested 

 by fixed forms. The latter may represent either alteration in length, or bendings, 

 twistings, or twinings, as illustrated on p. 406 in Fig. 119. Owing to these 

 alterations in form a part at least of the organ takes up a new relationship to 

 others, or occupies a new situation. When the new situation shows a relation to 

 the direction of application of the stimulus we speak of movement as a tropism. 

 When, however, the stimulus is not applied in any definite direction, or when 

 the orientation of the organ shows no relation to it but is determined by the 

 activity of the plant itself, we speak of the movements as nastic. We will 

 begin by considering directive movements or tropisms, and then deal with bending 

 or nastic movements, endeavouring in each case to determine whether they are 

 due to growth or to turgor. 



Bibliography to Lecture XXXIII. 



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DE BARY. 1884. Morphologic u. Biologic d. Pilze etc. Leipzig. 



COPELAND. 1 896. Einfl. d. Temperatur u. des Lichtes auf d. Turgor. Diss. Halle- 



CORRENS. 1891. Jahrb. f. wiss. Bot. 22, 161. 



DARWIN. 1877. Die Befruchtung d. Orchideen. 



DUTROCHET. 1 837. Mem. pour servir a 1'histoire des vegetaux etc. Paris. 1,451- 



EICHHOLZ. 1885. Jahrb. f. wiss. Bot. 17, 543. 



FISCHER, H. 1898. Cohn's Beitr. z. Biologic, 8, 53. 



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HILDEBRAND. 1873. Jahrb. f. wiss. Bot. 9, 235. 



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