304 



?,OTANY 



PART I 



a change in the position of the phototropic organs. The apical ends 

 of many positively heliotropic organs will be found to take up the 

 same direction as that of the rays of light. 



The exactness with wliich tli 

 with nioholus crystallinus (Fig. 

 quickly produced on moist horse 

 and turn their black sporangia 

 sporangia are shot away from the 

 the centi'e of the glass covering 



is is done is illustrated by an experiment made 



229). The sporangiopliores of this Fungus are 



or cow dung. They are positively phototropic, 



towards the source of light. When ripe these 



plant, and will be found thickly clustered about 



a small aperture through which alone the light 



has been admitted ; a proof that 



the sporangiopliores were ail 



previously pointed exactly in 



that direction. 



Upon closer investiga- 

 tion of the manner in which 



the POSITIVE PHOTOTROPIC 



CURVATURE of an organ is 

 accomplished, it is found 



THAT THE SIDE TURNED 

 TOWARDS THE LIGHT GROWS 

 MORE SLOWLY, THE SIDE 

 AWAY FROM THE LIGHT 

 MORE RAPIDLY THAN WHEN 

 ILLUMINATED FROM ALL 



SIDES. This may be readily 

 shown by previously mark- 

 ing with Indian ink regular 

 intervals from one to two 

 millimetres aj^art on the 

 opposite sides of the organ. 

 After the curvature has 

 taken place the intervals between the marks will be found to be 

 nnich farther apart on the shaded side than on the side turned to 

 the light. As compared with the elongation under normal con- 

 ditions of groAvth, 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 observation of the 

 ink-marks, that curvature takes place only in the portions of 



STEMS still in PROCESS OF GROWTH, AND THAT THE CURVATURE IS 

 AS A RULE GREATEST WHERE THE GROWTH IS MOST VIGOROUS. 



Fig. 220. — Piloholus crystallinus (P), abjecting its sporangia 

 towards the light. G, Slieet of glass ; B, opaque case 

 with a circular opRning at F; M, vessel containing 

 horse-dung. (Cf. description in text.) 



It was foriiicrly liulicveil that the increased growth of tlic shaded side was pro- 

 ducinl by the beginning of etiolation, and tliat the <liminishcd growth on tlie illumin- 

 ated side was due to the retarding (ilfeet which liglit exerts upon giowth in length 



