GROWTH AND MOVEMENT 479 



in attaining these positions may involve curvature, lengthening, and 

 twisting of the petiole and even of the blade itself. 



Perceptive region. Perception in most cases seems to occur in the 

 blade, whence the excitation is propagated to the petiole, whose upper 

 parts grow for the longest time, and even after elongation has ceased 

 may be started into growth again by the light. In some cases, however, 

 the petiole itself may be sensitive to light, and may either cooperate with 

 the blade, or alone be responsible for both perception and curvature. 



The mechanism of perception has been sought in the epidermis of the blades. 

 It has been found in some cases that the epidermal cells are domed and that they 

 act as lenses (fig. 699), focusing the 

 light upon the lower side of the cell, 

 so that a spot in the center is much 

 more brightly illuminated when the 

 light strikes at right angles. The 

 position of this area is shifted when 

 the leaf blade is oblique to the rays. 

 Correspondingly, it is assumed that FIG. 699 Ordinary epidermis and "ocella" 



. . . ; (o) of leaf of Dwscorea. After HABERLANDT. 



the protoplast is excited when the 



bright spot rests on any but the central area. There is no doubt that the structures de- 

 scribed concentrate the light, for that can be shown photographically ; but there are 

 sensitive blades in which domed epidermal cells are wanting, and experiments do 

 not yet unequivocally sustain the assumed distribution of irritability. The per- 

 ceptive organs of leaves have not been located other than by this still doubtful 

 hypothesis. 



(7) Other tropisms with radiant energy 



Electrotropism. Currents of electricity passing through the medium in which 

 plants are growing, and presumably through the organs themselves, evoke various 

 curvatures according to the density of the currents used. By nature roots lend them- 

 selves especially well to experiment. Some of these responses, and possibly all of 

 them, are due to one-sided injury of the roots. The effects appear to be due to elec- 

 trolysis of the solutions used ; but whether by the direct action of the ions outside 

 or by the withdrawal of ions from the protoplast is not certain. Electrotropism or 

 galvanotropism may therefore be hardly more than a special form of chemotropism. 

 It does not seem likely that such stimuli act to any important extent in nature. 

 The more important effects of galvanic and static currents upon development have 

 already been described (see p. 438). 



Thermotropism. Thermotropism is also of little importance. Both roots 

 and stems of particular plants turn toward or away from a blackened plate radiating 

 heat, according to the temperature. In a similar way roots growing in sawdust 

 will grow toward or away from a source of conducted heat. Neither form of 

 reaction can be of much importance in nature. 



The same may be said of reactions to radium and its salts, as well as those to 

 X-rays. The injurious effects of these are more pronounced than the tropisms. 



