276 



BOTANY 



The exactness with which this is done is illustrated l>\ an experiment made 

 with Pilololits criisfiflfimtx 'Fig. 224). The sporangiophorcs of this Fungus arc 

 quickly produced on moist horse or cow dung. They arc positively heliotropic, 

 and turn their black sporangia towards the source of light. When ripe these 

 sporangia are shot away from the plant, and \\ill be found thickly clustered about 

 the centre of the glass covering a small aperture through which alone the light has 

 been admitted : a prool that the sporangiophores were all previously pointed 

 exactly in that direction ( 8I ). 



Upon closer investigation of the manner in which the POSITIVE 

 CURVATURE of an organ is accomplished, it is found TH.VI 



THE SIDE TURNED TOWARDS 

 THE LIGHT GROWS MORE 

 SLOWLY, THE SIDE AWAY 

 FROM THE LIGHT MORE 

 RAPIDLY THAN WHEN ILLU- 

 MINATED FROM ALL SIDES. 



This may be readily shown 

 by previously marking with 

 Indian ink regular intervals 

 from one to two millimetre.- 

 apart on the opposite sides 

 of the organ. After the 

 curvature has taken pl.-ue 

 the intervals between the 

 marks will be found to 1 it- 

 much farther apart on tin- 

 shaded side than on the 

 side turned to the light. 

 As compared with the 

 elongation under normal 

 conditions of growth, the 

 marks on the illuminated 



side have remained nearer together, while those on the shaded side 

 have drawn farther apart ; that is, the growth in the case of a positive 

 heliotropic curvature has been retarded on the illuminated side and 

 promoted on the shaded side. It also becomes evident, from observa- 

 tion of the ink-marks, THAT CURVATURE TAKES PLACE ONLY IN THE 

 PORTIONS OF STEMS STILL IN PROCESS OK GROWTH. AND THAT THE 



CURVATURE IS AS A RULE GREATEST WHERE THE GROWTH IS MOST 

 VIGOROUS. 



It was formerly believed that the increased growth of the shaded side \\as pro- 

 duced by the beginning of etiolation, and that the diminished growth on the illumin- 

 ated side was due to the retarding effect which light exerts upon growth in length 

 (p. 258). Other heliotropic phenomena were found to be at variance with thi-. 

 explanation of hcliotropisin. Unicellular perfectly transparent fungal hypha- are 

 also subject to positive hcliotropic curvature, although in this instance there can 

 be no shaded side: on the contrary, the side of a hypha turned away. from the 



Fit;. L'lM. r;ii,l,nhi* :i"!i.<t<tlliini.*(I'). a 



tonanis tin; li^lit. (!, Sheet nt' ^lass 

 with a circular npiMiin.L,' at 1- : M. v 

 hiirse.(liin<;. (Cf. (leseri]ition in text.) 



nx its s]Hiiaiii:i;i 

 /;. <i]mque case 

 ssel containing 



