828 
MECHANICS OF GROWTH. 
2. In the Arc-indicator the thread c f (Fig. 480) fixed to the plant a is carried over 
the pulley d and fixed to a pin which is attached to a second pulley^. An index % made 
of a straight and firm straw is fastened to this second pulley in the radial direction, its 
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 i which tends to turn the pulley in the opposite 
direction with a force which keeps the thread r^^in a state of tension. As the internode 
below the hook h lengthens, the weight / sinks, and a piece of the thread c f oi equal 
length is rolled off the pulley g, 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 Auxanometer gets rid of this difficulty. It consists of a simpler 
form of the instrument already described. The thread y fastened to the plant sets directly 
in motion the pulley which carries the index x, 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 than the other side. On the former side is fastened a piece of 
smoked paper p p. 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 
inscribes another line lower down. The distances between the lines described on the 
cylinder evidently depend on the rapidity of growth of the plant ^. When, in consequence 
of this growth, the index has, after say twenty-four hours, reached the lower margin of 
the paper p p, 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 tempera- 
ture and the hygrometric conditions, at least between morning and evening. Fig. 481 
shows in addition a tin vessel B, 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 2. It has already been shown 
^ See Arbeiten des Würzburg. bot. Inst., Heft II. [Wiesner, Ueb. eine neue Construction des 
selbstregistrirenden Auxanometers, Flora 1876.] 
2 F. Burkhardt in Verhandl. der naturf. Ges. in Basel, 1858, vol. II. i, p. 67. — Sachs, Jahrb. für 
■wissensch. Bot. i860, Heft II. p. 338. — Alph. DeCandolle in Biblioth. univ. et rev. Suisse, Nov. 1866. 
— H. de Vries, in Archiv, neerlandaises, 1870, vol. V. — Koppen, Wärme und Pflanzen-Wachsthum, 
Dissertation, Moskow 1870. [See also Haberlandt, Landw. Versuchsstationen, XVII, 1874; Just, 
Cohn's Beiträge, II, 1877; Uloth, Flora, i'^7i and 1875; von Höhnel, in Haberlandt's Wiss. prakt. 
Unters. II, 1877.] 
