HELIOTROPISM OF THE STIPE 123 



A fruit-body which is very young — so that its pileus is only 

 just becoming differentiated as a tiny cone on the top of the enlarging 

 stipe-base, the whole being not more than about one quarter of an 

 inch in length — appears to be not only ageotropic but also anheUo- 

 tropic. Experiment showed that, at this stage of development, 

 unilateral light, like gravity, is unable to bring about any tropic 

 response. The rudiment of the fruit-body appears simply to grow 

 outwards from the substratum in the direction of least mechanical 

 resistance. As the young fruit-body becomes longer and longer, 

 it becomes (as we have seen) responsive to the stimulus of gravity 

 and so makes a geotropic curvature upwards through the sub- 

 stratum. As soon as the pileus emerges into the hght, the stipe is 

 found to have undergone a physiological change so that, in weak 

 unilateral illumination, it gives a shght heUotropic response in the 

 manner already described. It is not improbable that the young 

 fruit-body becomes sensitive to the tropic stimulus of gravity sooner 

 than to that ol light. 



The slope of a stipe at an angle of only about 10° from the 

 vertical toward a window from which the light is striking the fruit- 

 body at an angle of 70° from the vertical is evidently a resultant 

 position due to the response of the stipe to two stimuli acting 

 simultaneously in different directions. Gravity has tended to 

 direct the stipe vertically upwards, while the light has tended to 

 cause the stipe to grow away from the vertical at an angle of about 

 70°. It is evident that of the two stimuli gravity is by far the 



Fia. 69— cont. 



vertical. C, the same fruit-body 24 hours later than at B : it has grown in size 

 and, in response to the stimuli of light and gravity, has bent toward the source 

 of the light through an angle of 24°, so that it is now sloped toward the light 

 at an angle of 8° with the vertical. This resultant position, taken up in response 

 to the action of two stimuli — that of light tending to turn the fruit-body into 

 a horizontal position and that of gravity tending to turn the fruit-body into 

 a vertical position — indicates that the response of the stipe to the stimulus of 

 gravity is greater than the response to the stimulus of light. D, the same 

 fruit-body 24 hours later than at C : it has grown to a height of eleven inches 

 and its pileus is expanding : the stipe, in its further growth after the stage 

 shown at C, gradually ceased to be positively heliotropic and came to respond 

 to the stimulus of gravity only, so that its upper end has become erect. Had 

 the fruit-body responded as strongly and continuously to the stimulus of light 

 as to the stimulus of gravity, it would have had a slope toward the light of 45° 

 with the vertical, in which case, owing to its considerable weight and the distance 

 of its centre of gravity from its base, it would have become insecurely fixed to 

 its substratum and would have toppled over. B and C, natural size ; D, 

 reduced to one-half the natural size. 



