December 23, 1915] 



NATURE 



M 



unchanged? To investigate this a leaf-stalk of 

 Geranium pratensc was split to within 8 mm. of the 

 apex. The unsplit part was then exposed to light, 

 the blade and the lower portion of the stalk being 

 kept in the dark. The result shows (Fig. 5) that the 

 stimulus received by the upper part of the stalk is 

 transmitted to both halves, and the posterior half 

 curves in the direction of the light. The end of the 

 posterior half is, however, coiled backwards as before. 



Fig. 5. — Geraniu 

 of the apex : 

 kept in the dark 



fitatense. Petiole split to within S mm. 

 d then expose! to light ; the lamina was 



If we split a stalk into four we get the same result 

 (Fig. 6) ; all the four separate parts of the leaf-stalk 

 curve quite distinctly to the light. 



Now arises the further interesting problem. If 

 the leaf-stalk is split right up to the apex, will any 

 effect of light be produced in the posterior half? A 

 leaf-stalk split up to the apex was immersed in water 

 for some time until a distinct spiral curvature was 

 produced in both halves. The upper 8 mm. of the 

 two halves were then exposed to light, one half being 

 in front of the other, the blade and remainder of the 

 stalk being kept in the dark. At the end of several 

 hours' exposure to light, not only was the anterior 

 half much coiled — due to the heliotropic stimulus and 

 turgescence of the pith acting together — but the pos- 

 terior half also showed a distinct curvature to the 

 light in the motor region (Fig. 7). We find, there- 

 fore, notwithstanding the fact that two halves of 



Kk;. 6.~GeraHium firaUnse.— Petiole iplit into four. All 

 four parts curved to the light ; the lamina was in the 

 dark. 



the percipient region, the anterior and posterior, are 

 cornpletely separated from one another, that the pos- 

 terior half receives a stimulus as well as the anterior 

 half, and that this determines in it a definite helio- 

 tropic curvature. Some attempt was then made to 

 determine (i) what tissues of the stalk are concerned 

 in the perception of light, and (2) the tissues through 

 which the stimulus is conducted. In the first place, 

 the epidermis of the stalk was completely removed and 

 NO. 2408, VOL. 96] 



the upper 10 mm. of the stalk then exposed to the 

 light, leaving the blade and the rest of the stalk in 

 [ darkness. After several hours a definite curvature to 

 the light was obtained, although not so pronounced 

 as in an uninjured stalk. This was probably due to 

 the rough treatment to which the stalk had been 

 submitted by scraping off the epidermis. The experi- 

 ment, however, shows that the epidermis is not 

 essential either for the perception or the transmission 

 of this stimulus. 



Another leaf was then taken and the epidermis 

 together with a part of the underlying cortex removed. 

 In this case also, when the upper part of the stalk 

 was exposed to the light, a definite curvature was 

 obtained. Another leaf had the epidermis and the whole 

 of the cortex removed, but in this case, even after 

 an exposure of three days, no definite curvature to 

 the light was obtained. These experiments indicate 

 therefore that the cortical tissues are those mainly 

 concerned in the perception and transmission of the 

 stimulus. Further, several leaves were taken and 

 transverse incisions were made on opposite sides of the 

 stalk so that the tissues were completely cut across. 

 Here also a distinct but not very pronounced curva- 

 ture to the light resulted. It thus appears that 



Kiu. T.^Gcraniuiii pia.ense. Tne petiole w.-is split up to 

 the apex. The posterior half as well as the anterior 

 half curved distinctly towards the light. 



although the perception of light is located in the cortex 

 the stimulus can be to some extent transmitted trans- 

 versally through the tissues, probably through the 

 parenchyma and pith. That the pith is not necessary, 

 however, was proved by splitting the leaf-stalk longi- 

 tudinally into two halves and then removing by means 

 of a sharp scalpel the whole of the pith, together with 

 some small portion of the vascular bundles. On 

 exposing the upper part of the stalk thus treated to 

 the light, a definite curvature was obtained in both 

 halves of the stalk. 



It appears, therefore, from these experiments that 

 the perception of light is located, probably, mainly in 

 the cortex, but that the transmission of the stimulus 

 may be conducted both longitudinally and transversally 

 through any of the parenchymatous cells of the stalk, 

 and that the motor response, although much more 

 definite and pronounced when the whole of the cortex 

 is present, can also take place when this is partly 

 removed. 



We may now ask, What is it that the leaf perceives, 

 the direction of the light rays or the difference of 

 intensity and the illumination on the two sides of 

 the leaf? We cannot answer this question decisively; 

 it is probable that both hypotheses are to some extent 



