HELIOTROPISM 475 



of the cell. As yet the essential basis, however, for this very ingenious theory 

 is still wanting. It is remarkable that these 'ocular' papillae are not present 

 on orthotropic organs nor on the highly sensitive seedlings of the Gramineae. 

 Further there is no evidence forthcoming in support of the view that the upper 

 side of the leaf only is capable of appreciating the light stimulus. The experi- 

 mental evidence in favour of the function of the ocular cells will scarcely stand 

 critical investigation.] 



We have now to inquire into the other processes which follow on perception 

 of the stimulus. We have already referred to the excitation, the transmission 

 of the stimulus, and finally the reaction. Meanwhile there is still a further 

 phenomenon which must be noted ; we saw that the plant must be able to compare 

 the different intensities of light on different sides, so that we are now met with the 

 question as to whether the plant compares the perception itself or the excitation. 

 It is possible to hold the view that it does neither, but that the case is of a 

 more complicated nature. Probably the light operates on each individual cell 

 as a stimulus and induces in each a reaction, at present unknown to us. If 

 these individual reactions be uniformly alike, then no further effect is produced, 

 but if they be dissimilar, this difference induces an heliotropic excitation. As 

 to the nature of excitation we know nothing, and as to its transmission also 

 very little is known. In ROTHERT'S (1894) experiments with grass seedlings, 

 it was possible to determine only that the excitation travelled towards the 

 base, following the path of the parenchyma. We are compelled to assume that 

 the intercellular protoplasmic threads are the immediate agents in the 

 transference of the excitation from cell to cell, but such a fibrillar structure as 

 that described by NEMEC (1901) cannot play any special part in the transmission 

 (HABERLANDT, 1902). Finally, as to the last link in the chain, viz. the reaction, 

 the immediate cause we are at least acquainted with, and that is either 

 differential growth or differential turgor-stretching, 



We see, therefore, that our knowledge of the very important problems 

 which heliotropism presents to us is as yet very meagre ; many of these questions 

 lend themselves to experimental treatment and possible solution, and these 

 may in turn throw unexpected light on the remainder. 



Bibliography to Lecture XXXVI. 



BERTHOLD. 1882. Jahrb. f. wiss. Bot. 13, 569. 



DE CANDOLLE. 1832. Physiologic vegetale, 3, ioS6. 



CORRENS. 1892. Flora, 75, 87. 



CZAPEK. 1898 a. Jahrb. f. wiss. Bot.' 32, 175. 



[CZAPEK. 1898 b. Flora, 86, 425.] 



[DANDENO. 1903. Science, 18, 604.] 



DARWIN. 1881. Bewegungsverm6gen. German ed. by CARUS. Stuttgart. 



FIGDOR. 1893. Sitzungsber. Wiener Akad. Math.-nat. Kl. 102, I. Abt. 45. 



FRANK. 1 870. Die natiirl. wagerechte Richtung von Pflanzenteilen. Leipzig. 



HABERLANDT. 1901. Biolog. Centrbl. 21, 369. 



[HABERLANDT. 1905. Die Lichtsinnesorgane d. Laubblatter. Leipzig. See also 



Ber. d. bot. Gesell. 22, 105.] 

 HANSGIRG. 1890. Ber. d. bot. Gesell. 8, 353. 

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NEMEC. 1901. Reizleitung und reizleitende Strukturen. Jena. 

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