14S 



KNOWLEDGE. 



April, 1911. 



position, but he did not show any relation between the position 

 of stem and root and the resultant of the two forces involved — 

 gravity and centrifugal force ; that is. Knight showed that at 

 least in part the* so-called geotropic stimuhis is the gra\ity 

 stimuhis. but he did not show that the gravity stimulus is the 

 only stimulus involved. Giltay devised a special centrifuge in 

 order to test whether the position taicen up by the root and stem 

 is in reality the resultant of the two forces, andfound that this is 

 the case, with slight deviations that could be accounted for bv 

 variations in the speed of rotation and the variation of the roots 

 themselves. Hence it may be assumed that the geotropic 

 stimulus is identical in Nature with the gravity stimulus and 

 with that of centrifugal force. 



Since ISSO, when the Darwins published (" Power of Move- 

 ment in Plants"') the results of their experiments on the 

 behaviour of decapitated roots, there has been a good deal 

 of controversy regarding the perceptive region of the root. 

 Czapek (Jahrb. xciss. Bot., 1895) caused the root-tip of seed- 

 lings to grow into a boot-shaped glass cap, and showed that 

 when the terminal portion (1.5 millimetres long) of the capped 

 root was placed horizontally, the portion outside of the " boot " 

 curved so as to bring the root-tip into the position of equili- 

 brium — the vertical position. On the other hand, if the seed- 

 ling was fixed so that the tip was vertical and the rest of the 

 root horizontal, no curvature took place, the root simply con- 

 tinuing to grow without changing the position of the tip or of 

 the elongating zone. These results were interpreted to mean 

 that only the apical one or two millimetres of the root was 

 sensitive to gravitation, and it was generally thought that 

 the matter had been finally settled. The simple experiment 

 of cutting off two millimetres of the root tip, though it 

 destroyed the sensitiveness of the root, had been objected 

 to on the ground that the wounding might have also 

 destroyed the sensitiveness of the elongating zone behind 

 the wound, but Czapek's ingenious experiment was regarded 

 as final, though Jost and a few other writers have steadily 

 maintained that the question is still open. Newconibe 

 [Bcili. Dot. Ccntralb., Band xxlv.. Abt. ;.), after 

 upholding these objections and pointing out that " neither 

 Czapek's nor any other method so far employed has or can 

 prove the restriction of the perceptive region to the apical two 

 millimetres of the root," proceeds to show (1) that all the 

 phenomena observed accord equally well with the view 

 that sensitiveness extends through the entire growing zone, 

 but becomes diminished from the apex backwards, or 

 the view that sensitiveness is uniform through the growing 

 zone, but the tendency to automatic curvature — autotropism — 

 is stronger in the hinder than in the apical region ; (2) experi- 

 ments on the centrifuge with decapitated roots show that 

 geotropic sensitiveness is present more than four millimetres 

 distant from the tip. It must be admitted, of course, that in 

 a question of this kind the same results may be interpreted in 

 diametrically opposite ways by different observers, but it 

 would certainly appear that we should keep an open mind on 

 the matter. Newcombe's paper is of importance as a 

 reminder that even the most fundamental questions relating 

 to reflex actions in plants are by no means settled yet, and 

 that much further work is necessary before it will be possible 

 to obtain a clear picture of what happens in the growing tip of 

 a root when it emerges from the seed and grows down into the 

 soil. 



Another aspect of geotropism has just been re-in\estigated 

 by Nienburg {Flora, Band 102, 1911) in connection with the 

 movements of twining stems. Nienburg's work, like that of 

 Newcombe on the root, was done largely with centrifugal 

 apparatus, and he also has obtained results at \-ariance with 

 those of pre\'ious investigators. In 1881, it was shown by 

 Schwendener that the rotating movements of twiners like Hop 

 or Convolvulus, are not made by the stem when the plant is 

 kept revolving on a klinostat, and later it was suggested by 

 Noll that twining stems make a peculiar response to the 

 stimulus of gravitation, in that growth is promoted on 

 one flank, instead of on the upper side as in a root, 

 or the lower side as in an ordinary stem. This response 

 was termed " lateral geotropism," its result being a 

 revolving motion of the shoot apex. The clasping of supports 



by tendrils, like those of Peas or Vines, is, of course, an 

 entirely different phenomenon, due to contact irritability. 

 From his experiments. Nienburg concludes that all the facts 

 observed in the growth of twining plants can be explained as 

 due to the combined action of autonomous rotation (nutation) 

 and ordinary negative geotropism. and that neither Noll's 

 experiments nor those of later writers have established the 

 existence of any such thing as " lateral geotropism." However, 

 it would appear that here again we ha\e an as yet unsettled 

 question, and one which requires further investigation 

 with the aid of klinostat, centrifuge, and other methods of 

 experimentation. 



The curious "peg" or "heel" that grows out from the 

 seedling of Cucumber, Marrow, and other Cucurbitaceae, and 

 which holds down the seed-coat and helps the yoimg shoot to 

 escape, has been recently investigated carefully by Crocker, 

 Knight, and Roberts {Bof. Gaz., November, 1910). Accord- 

 ing to Francis Darwin, Cucurbit seedlings allowed to germinate 

 on a slowly-rotating klinostat produce pegs completely 

 surrounding the young shoot, and therefore appearing like a 

 collar, and he concluded that gravity determines the lateral 

 development of the peg, and that therefore this experiment 

 shows that gravity is continually effective on the klinostat and 

 is simply equalised in its action on the several flanks of the 

 rotated object : he also used the peg as a support for the 

 memory theory of plant response, assuming that its develop- 

 ment and position are directly determined by gravity. Crocker, 

 Knight, and Roberts, as the result of many experiments, 

 conclude that there is no evidence that gravity acts as a direct 

 stimulus to the lateral development of the peg, or that it leads 

 to increase in size of the peg ; that if the young shoot 

 (hypocotyl) is prevented from arching, the peg develops 

 equally all round ; that the lateral development of the peg is 

 simply brought about by the arching of the hypocotyl, the 

 most effective factor in this arching being the contact of the 

 seed-coat. 



CITRIC FEKMl'NTATION.— Since Wehmer in 1893 

 described the process of citric fermentation and showed that it 

 is due to a mould-like fungus, Ciiromyccs. several workers 

 have dealt with the subject, and Wehmer has recently given 

 a critical summary of their papers and of his own further 

 researches on citric acid fermentation, and the Citromycctes 

 \Zcitschr. f. Bot., Heft 2, 1911). The process is evidently 

 in the main one of oxidation of sugar — especially of malt, cane, 

 and grape sugars — and under favourable circumstances as 

 much as fifty per cent, of the sugar may be converted into 

 citric acid. The three sugars named yield the largest pro- 

 portion of the acid when acted on by the fungus, but a fair 

 yield is obtained from glycerine (nearly thirty per cent.), and 

 much smaller quantities or merely traces from various sugars, 

 inulin, alcohols. Up to the present it is doubtful whether the 

 action is caused by an enzyme produced by the fungus, for 

 negative results have been obtained with expressed sap and 

 with killed fungus. Many interesting questions I'egarding the 

 mode of action of the citric acid fungus arise from recent 

 investigations. It has been suggested that since acid is 

 formed, though sparingly, in the absence of free oxygen, the 

 first step in the action must be the splitting of the sugar 

 into carbon dioxide and alcohol — as in alcoholic fermentation. 

 The alcohol would then be oxidised with the production of 

 citric acid, this second action or citric fermentation proper 

 corresponding with the analogous processes of lactic and acetic 

 fermentation. 



R1:CI-:NT work on the GNFTALES.— The remark- 

 able group Gnetales includes the three genera Gnctiiin, 

 Ephedra and Welicitschia. which differ from all other Gymno- 

 sperms in having compound inflorescences, a long micropylar 

 tube, and true vessels, and which make a further approach to the 

 Angiosperms in the fact that except in Ephedra the archegonia 

 are reduced to isolated cells. Various writers have suggested 

 that the Gnetales form a transitional group between Gymno- 

 sperms and Angiosperms, though it seems more likely that they 

 had a common ancestry with the Angiosperms, and developed 

 parallel with them. Porsch (Ber. deutsch. bot. Ges., 1910) 



