450 TRANSFORMATION OF ENERGY 



organ is laid down but also in increased autotropism. As already mentioned, 

 Baranetzky (1901) assumes other factors still to account for the rest position 

 of the lateral organs, e.g. the influence of their own weight; into these questions 

 we cannot enter beyond saying that in our opinion a complete explanation of 

 the position of branches is not afforded by including the factors to which Bara- 

 netzky draws attention (Noll, 1902 ; [Pfeffer, Phys. II, p. 682]). After 

 these statements it would seem legitimate to inquire whether on the 

 whole there is such a thing as plagiotropism without correlation ; the case 

 of horizontal rhizomes might be alluded to, but it appears to us in the highest 

 degree probable that plagiotropism in these structures also is due to correla- 

 tions. We shall presently learn, however, that plagiotropism may be induced 

 by external factors without correlation. 



Relationship to the chief axis is not the only factor affecting geotropism ; 

 in the shoot itself the capacity for curving is in many cases influenced 

 by correlations. Miehe (1902) has shown that absence or incapacity of the 

 apex in Tradescantia stops or retards the curvature of the basal nodes. 



Having now glanced at internal factors we have still to consider external 

 influences which alter, often very greatly, the geotropic reaction. The first and 

 most important of these is temperature. Sachs (1874) found that an increase 

 in temperature decreased the angle between lateral and main roots ; more 

 remarkable alterations still have been noted by Vochting and more recently 

 by LiDFORS. These authors show that under the influence of a low tempera- 

 ture (a few degrees above zero) normally orthotropic shoots become plagiotropic. 

 The shoots of Senecio vulgaris in the wild state behave in this way at the 

 beginning of winter, as do also those of Sinapis arvensis (Vochting, 1898), 

 Holosteum umhellatum (Lidfors, 1902), and artificially etiolated shoots of the 

 potato (Vochting, 1902). The low temperature operates uniformly on all 

 sides on the shoot, and the plagiotropic rest position cannot, therefore, be 

 regarded as a resultant of two directive forces ; gravity is the sole directive 

 agent in this case, but temperature affects the nature of the response on the 

 part of the plant, the ' disposition ', as one might say. The cases investigated, 

 first by Stahl (1884), of the influence of light on plagiotropic organs may be 

 explained in the same way. A certain intensity of light induces plagiotropic 

 organs to exhibit positive geotropic movements ; thus, illuminated rhizomes of 

 Adoxa and Circaea bury themselves more or less vertically in the soil, not that 

 they turn away from light because they have become negatively heliotropic 

 (Lecture XXXVI), but because they have become positively geotropic, as 

 may be proved by a simple experiment. This alteration in disposition is of 

 the greatest service to the plant, since it prevents the rhizome from growing 

 out of the soil if it lives in suitable surroundings. Other rhizomes certainly 

 react quite differently, and yet quite in accordance with the necessities of the 

 case, for they exhibit negative geotropism when illuminated, and at the same 

 time alter into leafy shoots. Lateral roots behave in the same way as the 

 rhizomes of A doxa, for they, on illumination, make far smaller angles with the 

 chief root than they do when grown in the dark (Fig. 140). 



The medium also in which the root grows is of great importance in deter- 

 mining the reaction of the lateral roots ; Sachs (1874) found that the rest position 

 of such roots was not the same when cultures were made in soil as when made in 

 water or damp air. The greatest difference appears between the behaviour of the 

 side-root in the soil and in the air ; it stands out prominently on the main root 

 (Elfving, 1880 b). If the root be placed in the soil with its apex directed 

 upwards, a vigorous curving sets in, which almost always results in the reinsti- 

 tution of the normal direction of growth ; if the same experiment be carried 

 out in moist air the downward curvature is only slight and the apices of the roots 

 grow more or less horizontally. Obviously culture in damp air either weakens 

 the geotropic reaction or strengthens autotropism ; at all events, by supplying 



