154 GERMINATION, GROWTH, TISSUE TENSION. 



meters, or measured and recorded automatically at hourly or other 

 short intervals (auxographs, or self-recording auxanometers). 



In most of these instruments (Fig. 38) there is a wheel or pulley 

 over which passes a cord attached by one end to the stem or flower- 

 stalk (special devices are necessary in the case of roots and leaves), 

 the other end carrying a small weight to keep the cord taut. The 

 wheel is fixed above the plant, and the growth in length is observed 

 in various ways : (1) The descent of the weight equalling the elon- 

 gation of the plant is read off daily on a graduated scale fixed 

 vertically alongside it ; (2) a simple apparatus for magnifying the 

 movement is afforded by attaching to the wheel a light pointer which 

 moves over a graduated arc or a disc of cardboard. 



Either of these simple auxanometers can easily be made, the arc- 

 pointer form being useful for demonstration during a lesson : rapidly 

 growing stems should be used, e.g. the young flowering stem of a 

 sprouting Narcissus bulb. 



203. Influence of External Factors on Growth. 



Observations on growth e.g. auxograph records show 

 that there are great variations in the rate of growth. These 

 are largely due to changes in the varying external condi- 

 tions, of which the most important are (1) temperature, 

 (2) light; others are (3) humidity of the air, (4) -water 

 content of the soil. 



The influence of food supply on growth is readily seen on 

 comparing the growth of Bean seedlings from (a) seeds 

 with both cotyledons removed, (6) seeds with one cotyledon 

 removed, (c) intact seeds (see 191, 192) ; or of Wheat 

 seedlings from (a) grains with endosperm removed, (fe) in- 

 tact grains. This leads to the consideration of energy 

 supply, and this again is connected with respiration. To 

 study the dependence of growth upon respiration, we may 

 simply compare the growth of similar organs (a) when sup- 

 plied with oxygen, (b) when deprived of oxygen ( 205). 



The consideration of the pressure exerted by growing 

 parts ( 190) as the result of turgescence set up by osmosis 

 leads naturally to an important aspect of the relation of 

 osmotic pressure to growth namely, the relative tensions 

 of the tissues in growing steins and roots ( 207-210). 



204. Influence of Temperature on Growth. For exact 

 work it is necessary to use instruments by which a constant tempe- 

 rature may be maintained. This is done by means of thermostats 

 constant-temperature chambers or ovens. (1) We may expose a 



