^^6 LECTURE XXXII. 



another without exact investigation of the causes of growth. Tliey show, more- 

 over, how difficult and indeed impossible it is to infer the influence of any single 

 co-operating factor (temperature, light, moisture, grand-period,' periodic variations) 

 from observations made in the open air or in green-houses, where all the causes of 

 growth are at the same time subject to continual and violent variations. 



' The object of earnest research in this direction must on the contrary be simply 

 this — to study the effect of each individual cause of growth in detail by itself: 

 from this the common and natural course of the phenomena may then be analysed, 

 combined, and predicted more exactly than was previously possible.' 



Since 1869 I had been much occupied with preliminary investigations for the 

 measurement within short periods of the growth even of ordinary rather slowly 

 growing-plants, by means of a new apparatus. Of those described in the treatise 

 quoted I will here however only shortly describe the one represented in Fig. 348, 

 which I have designated the self-registering Auxanometer, since soon after my 

 publication this instrument became the model for numerous other instruments for 

 the measurement of growth, of which I consider only the one constructed by 

 Baranetzky as an actual improvement. 



The apparatus is especially for the purpose of observing the growth in length of 

 a sufficiently vigorous stem, in such a way that (i) the actual elongations are magnified 

 for the purposes of observation, (2) a writing or registering of the growth in each hour 

 is accomplished by the apparatus itself. Of course it is only possible after considerable 

 practice, and with trained hands, so to use such an apparatus that it yields scien- 

 tifically useful results. 



On a firm table stands a solid iron support A, on which by means of a movable 

 cross-bar the pulley r is movable : an indicator 2, consisting of a straw, is fastened 

 to the pulley. The plant y, grown in a pot, is in this case surrounded by a suitable 

 receptacle B, which can be opened in two halves and has a hole at the top for 

 the thread : this receptacle is for the purpose of keeping the growing plant in the 

 dark. Beneath the bud of the plant a pliant silk thread is attached by means of a 

 loop, and this is hooked on to a piece of thin wire, which is again connected with a 

 pliant silk fibre above : this thread passes over the groove in the pulley r. The small 

 weight g hanging on the pulley serves to give the coupling an equal tension. 



It is now clear that if the plant / becomes slightly elongated the pulley r will 

 revolve a little towards the left, and the apex of the indicator z therefore describes a 

 proportionately larger path : for example, if the indicator is twelve times as long as 

 the radius of the pulley, as was usually the case in my apparatus, then if the plant 

 becomes elongated i mm., the indicator must describe a path of 12 mm., and thus 

 the growth is proportionately magnified for the observer. The apex of the indicator 

 might of course be allowed to play on an arc of a circle of equal radius, and the 

 successive growths be observed on the graduations of this arc. To spare the 

 observation hour by hour, and for other advantages, however, I employed the 

 hollow cylinder C, made of stout sheet metal, and so fixed on the upright axis a 

 of the pendulum clockwork JD that it can be easily taken off : zu is the weight and 

 / the pendulum of this clockwork, which is for convenience so an-anged that the 

 cylinder C makes one complete revolution in exactly one hour, and that the clock- 

 work needs to be wound up only once a day. Previous to the observation a 



