!72 



NA TURE 



[April 16, 1903 



to plants; the function of statoliths is believed to be per- 

 formed (in Phanerogams, at least) by starch grains which 

 are free and movable, and thus fall to the lower end of the 

 cell. So long as the plant is vertical, the starch grains rest 

 in a layer on the basal walls of the cells. If the plant is 

 placed obliquely or horizontally, the falling starch grains 

 rapidlv take up a different position, and, by pressing on 

 a new region of the cell walls, can be conceived to originate 

 a stimulus. 



The fact that the power of being gravitationally stimu- 

 lated occurs in certain definite regions (e.g. the root-tip) 

 suggests the existence of that type of physiological machine 

 which we call a sense-organ. Now falling starch grains 

 supply the physical conditions which are known, in the 

 case of animals, to supply a sense-organ for orientation. 

 Therefore, when we find in the root-tip groups of specialised 

 cells provided with falling starch grains, such grains being 

 absent in the parts of the root which have no power of 

 geotropic perception, we have strong a priori evidence for 

 the statolith theory. 



This general line of argument has been fully and con- 

 vincingly developed by Haberlandt and Nemec, who have 

 also supplied direct experimental evidence. Some of the 

 latter is not quite so satisfactory. Thus Nemec succeeded 

 in destroying the starch in bean roots by embedding seed- 

 lings in gypsum, when such roots were found incapable of 

 geotropic curvature. Nemec not unnaturally put down his 

 results to the loss of an integral part of the sense-organs. 

 But I have shown that grass seedlings, the starch of which 

 has been largely removed by exposure to high temperatures, 

 not only fail to respond normally (o gravitational stimulus, 

 but also to the stimulus of light. The loss of starch must be 

 looked at as a symptom of general inability to respond to 

 stimulation rather than as a loss of special sense-organs. 



In the autumn of 1901, feeling the unsatisfactoriness of 

 the available methods of attacking the problem, I devised 

 what was then a new method. 1 My point of view was that 

 if gravitational sensitiveness is a form of contact-irritability 

 (which must be the case if the pressure of the statoliths on 

 the plasmic membrane is the critical event), then it might 

 be possible to intensify the stimulus by vibration. I hoped, 

 by applying vibration in a vertical plane to a horizontal 

 seedling, to make the starch grains dance on the lateral 

 walls, and by such repeated blows on the protoplasm to 

 produce a more active geotropic response. 



The result was as I expected, the seedlings which had 

 been kept horizontal for from eight to ten minutes, 2 on a 

 tuning-fork vibrating in a vertical plane, showed about 44 

 per cent, more curvature than the control specimens. 



In order to make sure that the tuning-fork did not act 

 by merely increasing the general irritabilitv of the seedlings, 

 the experiment was repeated with vertical specimens ex- 

 posed to lateral illumination. In this case it was found 

 that the curvature of the vibrated plants was only 5 per cent, 

 more than that of the control specimens. We may therefore 

 conclude that vibration increases the geotropic reaction, but 

 does not materially affect heliotropism. This is precisely 

 what might be expected on the hypothesis that geotropism 

 is the result of tactile stimulation of the plasmic membranes 

 lining the lateral cell walls by means of starch grains. So 

 far as it goes, the method is therefore clearly confirmatory 

 of the statolith theory. Francis Darwin. 



ENTOMOLOGY A T OXFORD} 



THE second volume of the "Hope Reports" contains the 

 papers published by the workers in the entomological 

 department of the University of Oxford during the years 1S97- 

 1900, and it is a cause for much congratulation to see this 

 evidence of the very interesting and important work that is 

 being done under the direction of Prof. Poulton with the 

 valuable collections of tracheate arthropods possessed by the 

 University. 



1 Practically the same method has meanwhile been made use of by 

 Haberlandt, who has published the results in Pringsheim's Jahrb., 1903. 



- After being subjected to vibration, the plants were placed on a klino- 

 stat to prevent further gravitational stimulation. The curvature was 

 measured after several hours slow rotation. 



; " Hope Reports," vols, ii., Hi., 1900, 1902. Edited by Edward B. 

 Poulton. Oxford: Printed for private circulation by Horace Hart, 190T, 

 1903.) 



In the first paper, on mimetic attraction, by Dr. Dixey, 

 there is an important contribution to the subject which seems to 

 he a favourite one with the Oxford entomologists, namely, the 

 evolution of the patterns of the wings ol those butterflies that form 

 Mullerian associations. The whole theory underlying the work 

 of Dr. Dixey and his colleagues has, it is well known, met with 

 considerable opposition from several well-known entomologists 

 who have studied Lepidoptera in tropical countries, and it is 

 therefore a very satisfactory feature ol this volume to find 

 included in it a good report of the discussion that took place at 

 the Entomological Society in 1S97 at the conclusion of Dr. 

 Dixey's papers. 



The two papers on mimicry, by Prof. Poulton, which follow 

 contain many additional facts of importance, but as they are 

 not illustrated, they are rather difficult to follow for those who 

 have not a special acquaintance with the butterflies ; but Prof. 

 Poulton's interesting communication to the Linnean Society 

 entitled "Natural Selection, the Cause of Mimetic Resemblance," 

 illustrated by five plates and several figures in the text, is an 

 important contribution to knowledge which any zoologist may 

 read with advantage. The volume also contains some reports 

 on the experimental inquiry into the struggle for existence in 

 certain common insects, and the colour-relation between pupa? 

 of several species of butterflies and the surroundings of their 

 larva?. 



The third volume is mainly devoted to the investigations of 

 Mr. Guy Marshall and Prof. Poulton on the bionomics of South 

 African insects. In South Alrica entomologists have found 

 several excellent examples of those forms of mimicry which are 

 known as " Batesian " and " Mullerian " mimicry respectively. 

 It was clearly important to test experimentally the value of the 

 colours of these insects as a protection from their enemies. 

 This Mr. Marshall has done with results which areas interesting 

 as they are remarkable. The fact that Manlida? and spiders 

 exhibit unmistakable signs that certain species of Lepidoptera 

 are distasteful to them, but are unaffected by colours whether 

 warning or cryptic in character, suggests that birds and other 

 vertebrates are the principal enemies which have caused the 

 evolution of the colour patterns of these insects. The experi- 

 ments with living kestrels and the results of an examination of 

 the contents of the crops of a large number of wild birds go a 

 long way towards a proof of the importance of the colours of 

 both Lepidoptera and Coleoptera as a protection from their 

 avian enemies. These and other investigations of a similar 

 character, excellently illustrated by several plates, make up a 

 paper of singular interest. The opponents of the evolution theory 

 as applied to the colours of insects have a difficult task before 

 them when they attempt to explain away the results of the 

 experiments that are here recorded. 



Space does not permit us to refer more fully to the other 

 papers which appear in these volumes, but enough has 

 been said to show that a very important work is being 

 carried on in Oxford. The rows and rows of insects that the 

 labours of entomologists in many countries have brought 

 together in the Hope Museum are not only ticketed and 

 arranged in systematic order, but they are made to yield up 

 facts "which, when intelligently studied, have an important 

 bearing upon the current theories of evolution. But this is not 

 all. Work that is done in a museum only, valuable as it may 

 be, is of little account unless it stimulates to, and is supple- 

 mented by, experimental work in the field. That this is what 

 museum work does lead to in Oxford is one of the most pleasing 

 features of these volumes. S. J. H. 



MAGNETIC WORK IN MARYLAND, U.S.A. 



TN a second report on magnetic work in Maryland (Mary- 

 land Geological Survey, special publication, vol. v. 

 part i. pp. 23-98, the Johns Hopkins Press, Baltimore, 1902), 

 Dr. Bauer gives the results of the survey which he com- 

 menced in iSqb. In the earlier years the work was done 

 mainly under the direct auspices of the Maryland Geological 

 Survey, but subsequent to May, 1899, when Dr. Bauer 

 took charge of the magnetic department of the U.S. Coast 

 and Geodetic Survey, the Geodetic Survey contributed 

 materially to it. The result, in Dr. Bauer's words, is that 

 " Maryland now possesses the most detailed magnetic survey 

 of any country, with the exception of Holland," there having 

 been on the average one station to each 100 square miles. 



NO. 1746, VOL. 67] 



