444 TRANSFORMATION OF ENERGY 



exerted thereby, although accessory changes may first of all lead to this result. 

 In any case the first influence of the stimulus of gravity must be a purely 

 physical or purely chemical alteration in the protoplasm, which we may term 

 perception. Following on this perception comes an excitation in the protoplasm, 

 which is undoubtedly distinct from the mere perception of the stimulus, since it 

 may be cumulative, as we shall see in Lecture XXXVI. We know, further, 

 that it is not coincident with the preliminaries of bending, that it requires 

 different formal conditions, and may take place in a different part of the plant. 

 In the latter case the excitation must be transmitted. If we consider the trans- 

 mission of the excitation as the third phase then we must rank the final reaction 

 as the fourth. We are quite ignorant whether no perception or no excitation 

 occurs in the cases where no reaction is manifested, as, for example, when the 

 organs are in the perpendicular position ; still we must believe that some kind 

 of response takes place in the resting position also, but it must affect all parts 

 uniformly and hence bending does not occur. Since stem and root grow 

 just as quickly on a klinostat as in the erect position it cannot be concluded 

 that the reaction is absent under normal conditions, especially since we do not 

 know for certain whether stimulation occurs or not to plants on a klinostat. 

 Since, however, Phycomyces, Chara and branches of weeping trees grow more 

 slowly in the inverted position than in the normal, it follows that gravity acts 

 as a stimulus in these cases. 



It must also be noted that attempts have been recently made to solve 

 the problems of geotropism by chemical and histological methods. NEMEC 

 (1901) has shown that special rearrangements occur in the protoplasm of cells 

 which respond to the geotropic stimulus. Doubtless, we are here dealing 

 not with a primary effect of gravity but with complex stimulus-phenomena 

 which bear some as yet unknown relation to the obvious curvatures. These 

 phenomena are of great interest, however, because it was formerly suggested 

 that gravity in the first instance induced movements in the protoplasm. 



CZAPEK (1898, 1903) has discovered that certain chemical changes take 

 place in parts which have been geotropically stimulated, and he has succeeded in 

 proving that tyrosin is oxidized into homogentisinic acid. This oxidation always 

 takes place in the plant, but homogentisinic acid is formed more abundantly 

 after the geotropic stimulus has been applied, and apparently this increase 

 is dependent in some way on geotropism, although what the connexion is is by 

 no means clear. It cannot have anything to do with the perception of the 

 stimulus, since it takes place in heliotropic curvatures also, which presuppose 

 another sort of perception (Lecture XXXVI). If connected with the reaction 

 it cannot occur first in the root apex and must also be distributed unequally in 

 the zone of movement. The subject offers a wide field for experimental 

 research. [Compare CZAPEK, 1905.] 



Bibliography to Lecture XXXIV. 



ANDREWS. 1902. Jahrb. f. wiss. Bot. 38, i. 



BARANETZKY. 1901. Flora, 89, 138. 



CZAPEK. 1895. Jahrb. f. wiss. Bot. 27, 269. 



CZAPEK. 1898. Ibid. 32, 175. 



CZAPEK. 1900. Ibid. 35, 313. 



CZAPEK. 1902. Ber. d. bot. Gesell. 20, 464. 



CZAPEK. 1903. Ibid. 21, 243. 



[CZAPEK. 1905. Annals of Botany, 19, 75.] 



DARWIN, CH. 1881. Bewegungsvermogen d. Pflanze. German ed. by CARUS, 



Stuttgart. 



DARWIN, FR. 1903. Proc. Roy. Soc. 71, 362. 

 [DARWIN, FR. 1902. Jour. Linn. Soc. Bot. 34, 266.] 

 [DARWIN, FR. 1904. Rep. Brit. Assoc. Cambridge.] 



