1897.] Mr Darwin, On Farmer's method, etc. 339 



It is easy to show that the cessation of circulation is due to 

 want of oxygen by lifting the cover-glass with a needle and adding 

 a drop of fresh water, when the protoplasm will begin to stream 

 in a few minutes. The same result can — as in Farmer's experi- 

 ment — be produced by illumination. The length of the period of 

 illumination required to start the movement varies : in a prepara- 

 tion darkened for about 3 hours, in which the circulation has only 

 recently come to a stand-still two or three minutes' exposure 

 to incandescent gas light will revive the movement in two or 

 three minutes. When the period of darkness has lasted over 

 6 hours it may require 20 — 30 minutes. This clearly depends 

 on partial inhibition 1 of the assimilatory powers of the chloro- 

 plasts. If the period of darkness is still further prolonged it is 

 easy to reach a condition in which it is apparently impossible 

 to revive the circulation ; but my experiments not being especially 

 directed to this point I cannot say what might be the effect of 

 very prolonged illumination. It is to avoid the inhibition of 

 assimilation that I mount several leaves under one cover-glass, 

 so that the dissolved oxygen may be exhausted before the chloro- 

 plasts fall into this condition. 



The method here described seems to me likely to be of value 

 to teachers in putting an interesting experiment within the power 

 of all. And it is worth noting that Farmer's experiment has all 

 the educational value of Engelmann's bacterial method. For 

 it demonstrates: (1) The evolution of oxygen by green plants in 

 the light. (2) The connection between movement and respira- 

 tion and the fact that a chlorophyllous cell makes use for purposes 

 of respiration of the oxygen evolved within itself. (3) It 

 demonstrates in a rough way (what Boussingault showed to be 

 true for aerial plants) that assimilation can begin in the absence 

 of free oxygen. (4) It also demonstrates the inhibition of the 

 assimilating function produced by prolonged darkness. 



For this last-named purpose it must be shown by the addition 

 of oxygenated water that the failure to circulate does not depend 

 on any loss of power in the protoplasm. 



The method also serves well to demonstrate the effect of light 

 of different wave-lengths on assimilation. As a source of light 

 I use incandescent gas, separated from the microscope by a glass 

 trough through which cold water flows. For the different lights 

 I use Landolt's screen solutions 2 in fiat-sided glass vessels. In 

 this way the maximum effect in the red 718 — 639 /jl/m can easily 

 be shown. The circulation, as far as I have seen, does not begin 

 in green 3 light unless the experiment is made after not more 



1 See Ewart in Journal Linn. Soc. Vol. xxxi. 



2 Berichte D. Chem. Ges. 1894, Vol. 27 (3), p. 2872. 



3 The light is 540 — 505 /j.fi, i.e. it shows a narrow strip of bine. 



VOL. IX. PT. VII. 28 



