^^8 LECTURE XXXn. 



which is provided with a laterally directed style, is so arranged that the style rests 

 lightly just beneath the upper margin of the paper, and the clockwork is then set in 

 motion. As the cylinder revolves, the apex of the indicator describes a line s on the 

 blackened paper ; when the second revolution of the cylinder begins the plant has 

 grown somewhat during the hour, and the apex of the indicator has therefore fallen, 

 and thus the second line which is now described on the paper lies at a certain distance 

 below the first. In this manner, hour by hour, a new line is described, and it is clear 

 that the distances between these lines can be used for obtaining, by the aid of certain 

 geometric considerations, an exact determination as -to the successive hourly elonga- 

 tions of the plant. In order to attain this, the clockwork is finally stopped, and the 

 paper cut off from the cylinder, the observer having first seized the apex of the 

 depressed indicator and carried it upwards, across the paper on the cylinder, in order 

 to mark the path on which the measurements of the distances between the lines 

 are to be taken. Immediately on its removal the blackened paper is drawn through 

 a bath of a solution of colophony in alcohol, and then dried, when the necessary 

 measurements can be taken. If the diameter of the cyUnder is sufficiently large, 

 however, and the indicator long enough, the errors of measurement in my apparatus 

 are so small (as may easily be calculated), that they do not come into con- 

 sideration at all if the experiment is properly conducted in other respects. More- 

 over, in my treatise, I explained in detail all imaginable sources of error, which are 

 to be sought far more in the nature of the plant than in the apparatus, so that 

 subsequent observers who have given my apparatus a modified form have had the 

 means of knowing in advance what were the important points. 



With the Auxanometer "described, and other apparatus, I have for years past 

 made hundreds of experiments, and thousands of measurements, the results of which, 

 in spite of a few insignificant differences of opinion, have received confirmation 

 also from the careful observations undertaken by Baranetzkyin 1879, with (as already 

 mentioned) an actually improved apparatus ^. 



We started above from the question as to how growth proceeds by day and by 

 night, and Fig. 349 will afford the shortest answer to this question. Since the essential 

 points are mentioned in the explanation to the figure, I may confine myself simply 

 to remarking that the rapidity of growth of a normal healthy stem attains a 

 daily maximum in the morning soon after sunrise, that then the hourly elongation 

 during the course of the day diminishes until evening, again to increase in rapidity 

 as the darkness comes on, often even before sunset ; and this increase of growth 

 continues until after sunrise, when the maximum is again reached. In this con- 

 nection the fact indicated in the figure by the dotted line 3 z is especially interesting ; 

 it shows that while the temperature gradually falls during the course of the night, 

 the rapidity of growth on the contrary may simultaneously increase. The contrary 

 may also be observed, that while the temperature is slightly rising, a diminution 



* J. Baranetzky, 'Die tdgliche Periodicitat im Langenwachsthum der Stengel' (Mem. de I'acad. 

 imp. des sc. de St. P^tersbourg, Vile sdrie, 1879). There also is described and figured the improved 

 Auxanometer referred to. With respect to the view proposed by Baranetzky that the periodicity in 

 the dark is an after-effect of the preceding illumination, it is absolutely necessary to read carefully 

 the treatise itself, and particularly pp. 16-18, since his preliminary publication (Bot. Zeit., 1877, 

 p. 639) omits just the most decisive facts. 



