402 



MOVEMENTS. 



A very slender filament of glass, made bj' drawing out a thin 

 glass tube until it is no larger than a hair, is to be afHxed to the 

 tip of the root, stem, or leaf under observation ; this is easily 

 done by means of a quickly drying varnish, for instance shellac 

 dissolved in alcohol. In order to mark the path made by the 

 filament it is best to cement to the tip of the slender hair of 

 glass a very minute bead of black sealing-wax, "behind which 

 a bit of card with a black dot is fi.xed to a stick driven into the 

 ground. The bead and the dot on the card are viewed througli 

 the horizontal or vertical glass plate (according to the position of 

 the object), and when one exactly covers the other, a dot is made 

 on the glass plate with a sharply pointed stick dipped in thick In- 

 dia ink. Other dots 

 are made at short 

 intervals of time, 

 and these afterwards 

 joined by straight 

 lines. The figures 

 thus traced are an- 

 gular ; but if the 

 dots are made every- 

 one or two minutes 

 the lines are more 

 curvilinear, as oc- 

 curs when radicles 

 are allowed to trace 

 their own course on 

 smoked glass plates " 

 " Whenever a 

 great increase of the 

 movement is not re- 

 quired, another and in some respects a better method of obser- 

 vation is followed. This consists in fixing two minute triangles 

 of thin paper, about one twentieth of an inch in height, to the 

 two ends of the attached glass filament ; and when their tips are 

 brought into a line so that they cover one another, dots are made 

 as before on the glass plate."' 



1 It is very convenient to employ large "bell-jars, or hemispherical glasses, 

 as glass screens upon which to recorii the dots indicating the position of the 

 tip at any given moment. It must be remembered that in all these cases there 



Fig. 17R. Tracing, showing tlie conjoint circnmnutation of the liypocotyl and cotyle- 

 dons of BrasBica oleracea during 10 hours and 45 minutes. Figure reduced to one half 

 original scale. (Darwin.; 



