474 PHYSIOLOGY OF GROWTH. 



some five to eight hours. If now, simultaneously with the objects 

 mentioned, we expose seedlings of Avena to the light, after darken- 

 ing a length of say 3 mm. at the tip of their plumules by covering 

 with small caps of tinfoil (prepared by wrapping strips of tinfoil 

 round a wire of suitable thickness, and carefully nipping the end 

 of the tube so formed), then the nutation begins at the summit of 

 the unshaded part of the leaf, it proceeds downwards more slowly 

 than when the whole seedling is illuminated, and remains also 

 comparatively flat, 10-40. The lower parts of the leaf are cer- 

 tainly in themselves heliotropically sensitive, but their sensitiveness 

 as compared with that of the tip of the leaf is small. When the 

 tip of the leaf is struck by the light, and gives a vigorous helio- 

 tropic reaction, the stimulus is transmitted to the lower part of 

 the leaf, and induces there a far stronger nutation than would 

 result from its own 'sensitiveness alone. Darwin (Poicer of Move- 

 ment in Plants) also called, attention to these interesting relations. 



The transmission of stimulus from the tip to the base of the 

 sheath leaves can also be demonstrated in the following manner. 

 We cultivate oat seedlings in pots not quite filled with soil. 

 When the plumule has attained a length of about 1*5 cm., we 

 cover the seedlings with finely riddled dry soil, so that they only 

 project for a length of 3 mm. at the tip. The soil is perfectly 

 impermeable to light beyond a depth of 2-3 mm., and yet when 

 the seedlings are illuminated unilaterally the basal part of the 

 plumule curves owing to heliotropic stimulus conveyed to it from 

 the tip. In this experiment the heliotropic curvature of the 

 shaded part of the seedling is induced by the conveyed stimulus 

 alone ; under normal conditions the nutation of the basal parts of 

 the sheath leaf is the combined result of this propagated stimulus 

 and the direct heliotropic perceptivity of the organ. These ex- 

 perimental results are undoubtedly of great importance in inter- 

 preting heliotropic phenomena in general. The results of the 

 following experiments are not less deserving of consideration. 



The plumule of seedlings of Avena sativa about 1'5 cm. in 



portion to the vertical, are made by means of quadrants, on which radii are 

 drawn at intervals of 5. The mean of several readings must always be taken. 



f That at the close of heliotropic nutations the greatest curvature is found in 

 most cases just at the base of the growing region, and not by any means at any 

 particular time in the zone of strongest growth, is quite in harmony with the 

 statements of Sachs (Flora, 1873, and Lectures} respecting the geotropic nutations 

 of shoots. See also Muller-Thurgau, Flora, 1876, p. 90. 



