74 B MECHAXICAL LAWS OF GROWTH, 



free end pointing to a graduated scale on the arc of a circle m n. The equilibrium of the 

 index is secured by the small weight / which tends to turn the pulley in the opposite 

 direction with a force which keeps the thread cf'va a state of tension. As the internode 

 below the hook h lengthens, the weight / sinks, and a piece of the thread cf of equal 

 length is rolled off the pulley^, thus raising the index on the arc. If the index is, for 

 example, ten times as long as the radius of the pulley, the portion of the arc which it 

 will pass over represents ten times the increase in length of the internode. But since it is 

 not usually required to know the absolute amount of the increase but only the relative 

 amount in different times, it is sufficient merely to read off and compare the movements 

 of the index on the graduated scale. By this instrument we are able to measure very 

 small increments of growth ; but, like the first process, it has the disadvantage that the 

 observer must watch it during the whole time, which renders the investigation very 

 difficult, especially at night. 



3. The Autographic Auxanomeier gets rid of this difficulty. It consists of a simpler 

 form of the instrument already described. The thready fastened to the plant sets directly 

 in motion the pulley which carries the index 2, being fixed to it by a pin at r. The 

 tension of the thread caused by the index itself is still further increased by the weight g. 

 By this contrivance the point of the index falls as the stem grows below the point to 

 which the thread is fastened. By means of the clock-work D the cylinder C fixed upon 

 the vertical axis a is made to rotate slowly, the rotation being arranged by adjusting the 

 length of the pendulum / so that a revolution is completed in exactly an hour. The 

 cylinder is however fixed eccentrically on the axis «, so that during the rotation one side 

 describes a larger circle that the other side. On the former side is fastened a piece of 

 smoked paper pp. When the index is properly adjusted, its point touches the paper 

 and describes on it a white line s s during the rotation of the cylinder. But after the 

 rotation has continued for some time the index is no longer in contact with the paper 

 owing to the eccentricity of the cylinder, but becomes so again afterwards when it in- 

 scribes another line lower down. The distances between the lines described by the 

 cylinder evidently depend on the rapidity of growth of the plants When, in consequence 

 of this growth, the index has, after say twenty-four hours, reached the lower margin of 

 the paper pp, the clock-work is stopped, the paper removed and replaced by a fresh 

 piece, the index being again set by raising the pulley, and the observation repeated. 

 The lines on the blackened paper are fixed by a varnish of collodion and dried, and the 

 distances between them are proportional to the hourly growths of the internode. It is 

 clear that the apparatus not only magnifies the increments, but also records them in the 

 absence of the observer, which is very convenient, especially for observing the nocturnal 

 growth. It is however necessary even in this case for the observer to note the temper- 

 ature and the hygrometric conditions, at least between morning and evening. Fig. 451 

 shows in addition a tin vessel 5, consisting of two halves united by a hinge, which may 

 be used for shutting out the light from the plant, even after the thread has been attached 

 to it. At E the thermometer / is placed in a similar vessel near the plant. 



Sect. 19. — Effect of Temperature on Growth^. It has already been shown 

 in Sect. 7 that the life of a plant generally and its growth in particular is carried on 

 only within certain limits of temperature (in general between zero and 50° C), and 

 that each function has apparently in every plant its inferior and superior limits ; 

 so that, for example, the lovv'est temperature at which a plant of wheat can grow is 



* See Arbeiten des Wiirzburg. hot. Inst. , Heft II. 



2 F. Burkhardt in Verhandl. der naturf. Ges. in Basel, 1858, vol. II, i, p. 67.— Sachs, Jahrb. fiir 

 wissensch. Bot. i860. Heft II, p. 338. — Alph. De Candolle in Biblioth. univ. et rev. Suisse, Nov. 1866. 

 — H. de Vries, in Archiv. neeilandaises 1870, vol. V. — Koppen, Warme und Pflanzen-Wachsthum, 

 Dissertation, Moskow 1870. 



