502 



LECTURE XXXII. 



off, is now so fixed that the observer sees distinctly one of the erect hyphag 

 with its sporangium, and that its apex apparently touches one of the division 

 lines in the telescope. The clock-work is now set in motion : the plant on the 

 plate revolves once in exactly one hour, and then comes again into the field of 

 view of the telescope, and the growth which has taken place during one hour is 

 now marked by the displacement of the sporangium on the scale ; and so with each 

 successive revolution. The measurements made hourly on this species then give 

 an idea of the course of growth in the thin hyphae of this Fungus, each consisting 

 of a single vesicle only. Since these objects not rarely elongate 2-3 mm. in an 

 hour, while it is possible to measure less than o-i mm., we obtain an exactitude 

 of measurement quite sufficient for our purpose. It may now be asked, for what 

 purpose the revolution of the plant by means of the clock-work is necessary; this 

 is simply in order that the plants shall grow quite vertically upwards ; for they 

 would not do this if they were exposed to illumination on one side from a 

 window, but would bend over towards the window, and make any measurement of 

 their growth impossible. By means of the slow revolution, however, the observed 

 part of the plant is successively illuminated on all sides during the course of 

 the hour, and this acts exactly as if it received equal light from all sides. Hence 

 no heliotropic curvature takes place. I regarded it as an important advance 

 when, in the year 1873, I first hit upon the idea of entirely doing away with 

 the heliotropic curvatures, so exceedingly disturbing in observations on growth, by 

 means of such revolutions on a horizontal disc. 



It is of course obvious that the glass bell-jar C may be replaced by an opaque 

 vessel, or, what is better, on account of the susceptibility of the object towards the 

 atmospheric moisture, the glass vessel may be retained and an opaque receptacle (of 

 cardboard for instance) placed over it. In this way the small plant may be exposed 

 alternately for one or two hours to the influence of light or of profound darkness, or 

 different coloured glasses may be interposed to demonstrate the effect of lights of 

 various colours, and so forth. 



Fig, 351.— Tlie thick interrupted line shows the varying 

 rapidity of growth of a hypha of Miicor in alternate light and 

 darkness. The thin line marks the temperature. 



As an example of the course of such a series of observations I subjoin a table and 

 the corresponding curves drawn from observations made by Vines (Fig. 351). 



