398 LECTURE XVI. 



In conclusion we may endeavour to form some idea as to 

 the mode of action of light in retarding growth. The effect 

 of the tonic action of light in diminishing the rate of growth 

 is probably to be ascribed to an interference with the motility 

 of the protoplasm of the growing cells, for we know that 

 when the light is of medium intensity, irritability, as indicated 

 by heliotropic curvature for instance, is well-marked, while 

 the rate of growth is considerably lower than it is in darkness. 

 We may regard, then, the tonic action of light, manifested 

 in its retarding effect upon the rate of growth, as an in- 

 hibitory action, and as being due to the induction of a certain 

 degree of rigidity in the protoplasm, the rigidity being slight 

 at low intensity and gradually increasing with the intensity, 

 until, under the influence of very intense light, it is com- 

 plete. A detailed discussion of this point will be given here- 

 after when we come to the general explanation of all the 

 various phenomena of this kind. 



It will be advantageous to give some account of the methods by 

 which the measurement of growing organs has been made. It has been 

 made in some cases by simply using the rule ; in others, by observing 

 the elongation of the organ by means of some form of telescope, with 

 reference to a fixed scale either in the telescope or applied to the organ : 

 in other cases, again, by means of pointers connected with the organ and 

 playing over a graduated arc. The most convenient form of instrument 

 is the self-registering auxanometer, first devised by Sachs, in which the 

 point traces its rise and fall upon a piece of smoked paper fastened on to 

 a rotating drum. Appended is a figure of Baranetzky's improved form 

 of self-registering auxanometer. It consists of a table bearing the 

 rotating drum, and carrying on each side a stand, which will slide up 

 and down as required, for the plants ; in the figure only one plant is in 

 position. A thread is attached by one end to the top of the stem of the 

 plant, and has a weight at the other end to keep it tense. It passes over 

 a pulley, and on the same axle as this pulley there is a large grooved 

 wheel : over this wheel and the other above it there passes an endless 

 cord to which the indicator is attached, the point of the indicator being 

 in contact with the revolving drum. An elongation of the stem will 

 cause the weight at the end of the thread to sink, and will cause the 

 pulley to turn : this movement will be communicated to the grooved 

 wheel on the same axle, and will cause a movement of the indicator 

 which will be traced on the drum. The drum may be made to rotate 



