GEOTROPISM. I 435 



Differential growth has been estabhshed not only between the upper 

 and under sides of an uninjured and complete organ but also in isolated portions ; 

 thus in stems, at all events, curvatures have been observed in parts cut out 

 from a shoot, while roots, after wounding, remain for a longer time insensitive to 

 the influence of gravity. It is by no means remarkable that separated segments 

 ■of stems should still show geotropic curvature, but the behaviour of plant organs 

 when split longitudinally is worthy of note. If a stem be split longitudinally a 

 curving outwards of each half must occur in consequence of differences in tissue 

 tension, but if one of these halves be arranged so that its epidermis is upper- 

 most, and the other half be in the reverse position (the cut surface of the 

 medulla lying horizontally), geotropism influences each half differently, and 

 induces in them differential growth between the upper and under tissues ; in 

 the section which lies epidermis upwards growth in the medulla is accelerated 

 while the epidermis shortens, in the other half the epidermis increases in 

 length and the medulla grows less vigorously than in the other half. Tissue 

 tensions, however, in this case, to a certain extent, obscure purely geotropic 

 curvature. If similar researches are made with nodes of grasses, i. e. with 

 the swollen basal regions of the leaf sheaths where tissue tensions of this type 

 are absent, geotropic curvature may be determined both in the upper and under 

 longitudinal halves ; it makes no difference which side is uppermost. De Vries 

 <i88o) has shown that geotropic curvature occurs in each longitudinal area 

 even if the shoot be divided in four. 



Grass nodes are of interest from another point of view. In the organs 

 hitherto spoken of the geotropic curvature depends on longitudinal growth ; 

 where longitudinal growth ceases there curvature is also absent. At all events, 

 this is the conclusion to which all investigators have come who have examined 

 the question, with the exception of Kohl (1894), who holds an opposite view. 

 The nodes of Gramineae are able, however, to develop geotropic curvature in 

 the full-grown condition, for they are capable of renewing growth each time 

 they are removed from the position of geotropic rest. In this curvature 

 the under side undergoes great elongation ; in a very short time it becomes 

 double or even as much as five times as long as it was, while the upper side 

 is forcibly compressed so much as to throw it into folds. A few numerical 

 details (Sachs, 1872, 206) will make this clear. 



Cinquantino Maize. 



Length of nodes in mm. Upper. Under. Upper. Under. Upper. Under. 



Before bending 4-3 4-1 40 5-0 5-0 50 



After bending 25 90 30 ii-o 4-5 12-5 



Difference —1-8 +49 —10 +60 —0-5 +75 



More recentl}', from many points of view, it has been shown that not 

 only grass nodes and related structures, but many other full-grown organs, may 

 develop curvature when subjected to geotropic stimulus. Branches also which 

 are secondarily thickened may exhibit geotropic curvature, which cannot 

 be induced in plants, e. g. palms, which have no power of secondary growth. 

 It must be assumed that the power of curving rests in the power the cambium 

 has of producing elements of different lengths on either side. Detailed investi- 

 gations on these points are, however, not available. (Compare Meischke, 1899 ; 

 JosT, 1901 ; Baranetzky, 1901.) 



In every case which has been accurately studied the immediate cause 

 of the curvature is a difference in longitudinal growth of opposite sides. As 

 is generally the case a stretching of the cell-membranes due to turgor pre- 

 cedes surface growth, and this is gradually rendered permanent by growth. If 

 the organ be plasmolysed at the commencement of geotropic curvature it 

 grows at first in a straight line, later on, however, the curvature is permanent. 



F f 2 



