486 TRANSFORMATION OF ENERGY 



solutely saturated, but that the saturation is always increasing sufficiently 

 to induce directive movements on the part of the fungus. In the case of other 

 plants, however, grown on a klinostat, e. g. Lepidium sativum, orientation with 

 special reference to the substratum may easily be observed. If Lepidium be 

 cultivated on a cube of turf the plumules arrange themselves at right angles to 

 the four faces of the cube which are parallel to the axis of the klinostat, while 

 they assume somewhat oblique relations to the two other sides, turning their 

 convex sides to the axis. These positions cannot be explained by hydrotropism 

 since the plumules of Lepidium are not hydrotropic. Probably we have here 

 to deal with heliotropic movements which in the conditions of illumination 

 under which the experiment is generally conducted are by no means excluded 

 (DiETZ, 1880). In the dark such orientation in relation to the substratum com- 

 pletely disappears. 



We may conclude this lecture by drawing attention to two other tropisms 

 about which little is known. Darwin (1881) was the first to describe traumata- 

 tropism in roots. If the growing point (Spalding, 1894) of a root be injured on 

 one side by some chemical or by heat, a curvature takes place in the growing 

 zone, which has the effect of removing the end of the root from the injurious 

 substance. Data with regard to this tropism are almost entirely wanting [com- 

 pare Burns, 1904] ; we do not know whether the stimulus is to be sought for 

 in some chemical change in the root or whether even the curvature itself has any 

 purpose in nature. 



We are better acquainted with the phenomena of rheotropism first described 

 by JoNSSON (1883) as occurring in roots, more especially in those of seedlings, 

 but not limited to them. When such roots are growri in running water, they 

 exhibit a curvature in the opposite direction to the course of the flow (positive 

 rheotropism). Newcombe (1902 a) found that in the case of the roots of Cruci- 

 f erae, which respond best to this stimulus, the minimum rate of water flow which 

 was capable of acting as a stimulus was 2 cm. per minute ; the best results were 

 obtained when the speed had reached 100-500 cm., and at about a 1,000 cm. 

 negative curvature ensued, although these would appear to be due to purely 

 mechanical causes and not to be stimulation phenomena at all. Juel (1900) 

 has made similar experiments with like numerical results for Zea mais and Vicia 

 sativa. Rheotropism, however, is not a peculiarity exhibited by all roots, 

 and the individual variations in different cases are, according to the state- 

 ments of all investigators, very considerable (compare Berg, 1899). 



More recently (1900) Juel has shown that decapitated roots still react 

 rheotropically, and Newcombe (1902 b), considers it probable that the stimulus 

 makes itself felt not only in the growing zone but also at the apex and in older 

 parts up to a distance of 15 mm. from the apex. That rheotropism has nothing 

 in common with hydrotropism, as one might at first sight imagine it had, has 

 been shown experimentally by J uel, who has proved that the immediate percep- 

 tion is due to the pressure of the flowing water. Hence rheotropic curvatures 

 should be correlated with the movements in roots due to unilateral contact 

 (Lecture XXXVIII). 



We have now gained some insight into tropistic curvatures due to a large 

 number of different stimuli, but we must not attempt to consider the combined 

 action of several stimuli, as we attempted to do in the case of geotropism 

 and heliotropism. 



Bibliography to Lecture XXXVII. 



De Bary. 1884. Vgl. Morphologic u. Biologie d. Pilze. Leipzig. 

 [Bennett. 1904. Bot. Gaz. 37, 241.] 

 Berg. 1899. Lunds Univ. Arsskrift, 35. 

 Brunchorst. 1884. Ber. d. bot. Gesell. 2, 204. 

 [Burns. 1904. Beihf. bot. Centrbl. 18, I, 159.] 



