412 



NA TURE 



[August 27, 1891 



uniform thin membrane, the branching of Cladophora, and the 

 escape of sexual products in certain Algre. 



We now pass on to the work of two observers, Wortmann and 

 Noll, who have devoted special attention to mechanism of curva- 

 tures. Wortmann [Bot. Zett., 1887, p. 785) started on the 

 assumption, already several times mentioned, that the growth- 

 curvature of acellular and multicellular organs must have a 

 common cause. He began by testing Kohl's statement {Bot. 

 Hcfte, Marburg, Heft v. [I have not seen Kohl's paper]) that when 

 the sporangiferous hypha of a Phycomyces curves apogeolropically 

 or heliotropically, &c., there is a collection of protoplasm on the 

 concave wall. Wortmann principally investigated the curvature 

 discovered in Phycomyces by Errera {Bot. Zeitiivg, 1884) which 

 can be produced by contact. When the hypha is touched with 

 a glass filament or with a platinum wire, or by allowing a speck 

 of indian ink to dry on it, it curves over towards the touched 

 side. The hypha is so highly sensitive to contact that it curves 

 in from three to six minutes ; it is clearly a growth-curvature, 

 for it only occurs in the part of the hypha which is growing. In 

 curvatures thus produced, as well as in apogeotropic and helio- 

 tropic curvatures, the accumulation of protoplasm on the 

 concave side is, according to Wortmann, clearly visible, and, 

 what is more important, the membrane becomes thicker on the 

 concave side, sometimes twice as thick as on the opposite side 

 of the cell. In consequence of the unequal thickening of the 

 membranes, the cell is supposed to yield asymmetrically cell- 

 pressure, and the necessary consequence is that the cell grows . 

 into a curved form. 



In applying the same method of investigation to multicellular 

 parts, Wortmann fallowed Ciesielski(Cohn's " Beitrage," 1872,- 

 p. i), who noticed that in geotropically curved roots the cells of 

 the concave (lower) side of the organ are much more densely 

 filled with protoplasm than are the convex cells. Sachs 

 (" Vorlesungen," p. 842) describes a similar state of things in 

 the halms of grasses, and Kohl, again, in tendrils and the stems 

 of climbing plants. 



Wortmann first of all made sure that no redistribution of proto- 

 plasm could be observed in the individual cells of curving multi- 

 cellular organs. If each cell behaved independently like a free 

 cell, we might expect to find a collection of protoplasm on the 

 concave wall of all the constituent cells of a curving shoot. 

 But this is not the case. Nor at first could any microscopic 

 differences be made out between the concave and convex tissues 

 of a curving shoot. But when the stimulus was made to act for 

 a long time, differences were apparent. A young Phaseolus 

 plant was placed so that the epicotyl was horizontal and was 

 forced to grow in the horizontal direction by a thread attached 

 to the end of the stem, passing over a pulley and fastened to 

 a weight. Here the geocropic stimulus could continue to act 

 for 24-36 hours, and under such conditions a marked change in 

 the tissues was visible. The cells of the cortex on the upper 

 side became densely filled with protoplasm, while the lower cor- 

 tical cells were relatively poor in protoplasmic contents. The 

 same changes in the membranes occur as those noticed in Phy- 

 comyces — that is to say, the walls of the cortex on the upper side 

 are very much thicker than those on the lower side.^ 



Since the walls of the cortical cells have become more resisting 

 on the upper than on the lower side, then (assuming the osmotic 

 expanding force to be the same in both cases) the growth will 

 be quicker on the lower side, and the shoot will curve upwards. 

 Wortmann states that his observations account for the fact that 

 the convex side grows quicker, not merely than the concave, but 

 than a normal unbent shoot. But he does not seem to have 

 compared the thickness of the convex cell- walls with the normal, 

 although he states that they are poorer in protoplasm than is 

 usual, and from this it may, according to his views, be perhaps 

 assumed that the membranes are abnormally thin. 



Wortmann points out that his views account for two well- 

 known features in growth-curvatures, viz. the latent period ^n^ 

 the after-effect. If a curvature can only occur when a difference 

 in structure of cell-walls has arisen, it is certainly natural that 

 some time should occur before the curvature is apparent. I do 

 not lay much stress on this part of the subject, as I feel sure the 

 whole question of latent period needs further investigation. 

 With regard to after-effect it is true that Wortmann's views ac- 

 count for the continuance of curvature after the stimulus has 

 ceased to act, 



Wortmann attaches great importance to another point in his 



' Both protoplasmic change and thickening of cell-walls occur to some ex- 

 ent in the pith. 



NO. I 139, VOL. 44] 



theory, which, could it be established, would be of the greatest 

 interest, and would unite under a common point of view, not 

 only acellular and multicellular organs, but also naked proto- 

 plasm, e.g. the Plasmodia of myxomycetes. The view in ques- 

 tion was tentatively suggested by Sachs (" Lehrbuch," 1874; 

 Eng. trans., 1882, p. 841), and mentioned by Pfeffer (" Pflanzen- 

 physiologie," ii. p. 331) in a i-imilar spirit. The apogeotropic 

 curvature of a Phycomyces-hypha is supposed to be due to the 

 unequal thickening of the membrane on the upper and lower 

 sides, and this to be due to the migration of protoplasm from 

 the lower to the upper side of the ceil. In the same way in a 

 multicellular organ the pi^otoplasm is supposed to migrate from 

 the lower cortex and pith to the upper cortex and pith, such 

 migration being rendered possible Ijy the now generally ad- 

 miiied intercellular protoplasmic communication. Thus the 

 apogeotropism of a cell or a multicellular part would be due to 

 the apogeotropism or tendency to migrate vertically upwards of 

 the protoplasm. There are great difficulties in the way of ac- 

 cepting this attractive theory. 



Noll (Sachs's Arheiten, 1888, p. 530) states that when a 

 curved Phycoinyces-hypha, in which protoplasm has accumulated 

 in the upper (concave) side, is reversed so that the mass of proto- 

 plasm is below, it does not migrate upward again, as might be 

 expected. Moreover, he points out that in Nitella and in Bryop- 

 sis the circulating protoplasm continues in movement, and does 

 not accumulate in any part of the cell. Lastly, there seems, as 

 Noll points out, a difficulty in believing in the migration of proto- 

 plasm through the very minute pores by which the plasma 

 strands pass from cell to cell. There seems much probability 

 in Noll's view that the plasmic strands only serve for the pas- 

 sage of impulses, or molecular changes, and that they consist of 

 ectnplasm alone, not of the endoplasm which Wortmann de- 

 scribes as the migratory constituent of the cell. 



Wortmann's theory has been criticized by Elfving {Finska 

 Vet. Soc. Forhand., Helsingfors, Bd. xxx., 1888). The essence 

 of Elfving's paper is that appearances similar to those described 

 by Wortmann can be produced by curvatures not due to stimu- 

 lation. Thus, when Phycomyces is made to grow again>it a 

 glass plate it is mechanically forced to bend. Yet here, where 

 there is no question of stimulation, the plasma collects along tlie 

 concave side of the cell. Elfving concludes that the visible 

 changes are the result and not the cause of the curvature. Elf- 

 ving also produced curvature in Phaseolus by bending the apex 

 of the plant towards its base and tying in that position. Under 

 these conditions the convex side of the shoot showed the changes 

 described by Wortmann in geotropic plants. Here again Elf- 

 ving gives reason to believe that the thickening of the cell-walls 

 is a result, not of curvature, but of strain mechanically produced. 

 When a plant is prevented from executing an apogeotropic 

 movement it is clear that a longitudinal strain is put on ihe 

 upper (concave) side. But the longitudinal strain in Elfving's 

 plants is on the convex side. Therefore, if, as Elfving believes, 

 the visible changes are due to strain, they should, as they do, 

 occur on the convex side in his experiments, on the concave in 

 Wortmann's. 



Wortmann replied in the Bot. Zeitung, 1888, p. 469, and at- 

 tempted to explain how Elfving's results might be explained and 

 yet his own theory hold good. The reply is by no means so 

 strong as the criticism, and it must be allowed that Elfving has 

 seriously shaken Wortmann's argument. 



Somewhat similar criticisms have been made by Noll (Sachs's 

 Arheiten, 1888, p. 496). In the acellular plants, Derbesia 

 and Bryopsis, Noll studied growth-curvatures, and was quite un- 

 able to detect any thickening of the concave cell-walls, except 

 when the curvatures were very sudden, and in these cases the 

 result could equally well be produced by mechanical bending. 



Noll further points out what is undoubtedly a fault in Wort- 

 mann's theory — namely, that he explains the retardation on the 

 concave rather than acceleration on the convex side. This criti- 

 cism is only partially just, for though Wortmann's description 

 only shows a relative thinness of the walls on the convex side, 

 yet it is clear he believed there to be an absolute diminution of 

 resisting power on that side. 



Noll's experiments with grass halms show clearly that accelera- 

 tion of growth on the convex side is the primary change, rather 

 than retardation along the concave half. When the halms are 

 fixed in horizontal glass tubes, so that they are stimulated but 

 unable to bend, the lower half of the pulvinus forms an irregular 

 out-growth, increasing radially since it is not able to increase 

 longitudinally. 



