TRANSACTIONS OF SECTION U. 669 



Sachs * had already pointed out that attention should be directed to changes in 

 extensibility of cell-walls as an important factor in the problem. 



Wiesner, in his 'Ileliotropische Erscheinung-en,' - held that the curvature of 

 multicellular organs is due both to an increase of osmotic force on the convex 

 side, and to increased ductility ' of the membranes of the same part. lie repeated 

 De Vries' plasmolytic experiments, and made out the curious fact that in many 

 cases the curvature is increased instead of being diminished. lie attributed the 

 result to the concave tissues being more perfectly elastic than ductile convex 

 tissues, so that when turgescence is removed the more elastic tissues shorten most, 

 and, by diminishing the length of the concave side, increase the curvature. 



Strasburger, in his ' Zellhaute,' 1882, suggested that growth curvatures are 

 due to increased ductility of the convex membranes, and gave a number of instances 

 to prove that a change to a ductile condition does occur in other physiological 

 processes, such as the stretching of the cellulose ring in ffidogonium to a uniform 

 thin membrane, the branching of Cladophora, and the escape of sexual products in 

 certain algae. 



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

 devoted special attention to mechanism of curvatures. Wortmann^ started on the 

 assumption, already several times mentioned, that the growth-curvature of acellular 

 and multicellular organs must have a common cause. lie began by testing Kohl's state- 

 ment* that when the sporangiferous bypha of a Phycomyces curves apogeotropically 

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

 "Wortmann principally investigated the curvature discovered in Phycomyces by 

 Errera,^ 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-curva- 

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

 thus produced, as well as in apogeotropic and heliotropic curvatures the accumula- 

 tion 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 

 followed Ciesielski,' 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 ^ 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 protoplasm could b& 

 observed in the individual cells of curving multicellular organs. If each cell be- 

 haved independently like a free cell, we might expect to find a collection of proto- 

 plasm 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 diflPerences 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, diSerences 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 gentropic stimulus could continue to act 



' Lelirhuch, ed. 4, Eng tr. p. 835. 



' Wiener Sitzunijsh. vol. Ixxxi. 1880, p. 7 ; also in the Denlisclmften , 1882. 



' "Weinzierl, SUzungsh. Wien, 1877, showed that strips of epidermis taken off the 

 convex side of heliotropically curved flower-stalks of tulip and hyacintli were about 

 twice as extensible when stretched by a small weight, 7'5 grammes, as approximately 

 corresponding strips for the concave side. 



< Bot. Zeit. 1887, p. 785. 



' But. Hefte, Marburg, Heft v. [I have not seen KoM's paper.! 



* Bot. Zeitmig, 1884. ' Cohn's Beitrdgc, 1872, p. ]. 



• Vorlesungen, p. 842. 



