GROWTH AND MOVEMENT 



121 



which are converted into the layers of cellulose successively deposited. 

 It is thus apparent how important in the process of growth are the 

 conditions of turgor of the cell, and of the presence of available plastic 

 material. The turgor depends ultimately upon sufficient water-supply, 

 and upon the vitality of the protoplast, which again requires access 

 of free oxygen for its maintenance. 



The actual rapidity of growth of the shoot or root as a whole varies in 

 different individuals, and according to external circumstances. In 



14° 16 1£ 



20 2_2 24 26 28 30 32 34 36 38 40° 



Temp. 



Fig. 84 bis. 



Chart showing the relations of growth (in ram.) to temperature (in degrees Cent.), 



in the three seedlings named. The curves are similar, but the optimum, that is 



the crest of the curve, varies for different plants. It is higher for Maize and Melon 



than for Lupin. (After D'Arcj' Thomson, based on observational data of Sachs.) 



extreme cases it is sufficiently quick to be directly observed. Shoots 

 of the Giant Bamboo grow in length over a foot a day, a rapidity that 

 can be followed by careful observation with a lens. But as a rule 

 growth is so slow that it is not readily apparent. Various mechanical 

 arrangements by pulleys and levers have therefore been devised for 

 amplifying and measuring growth when it is slow. These are termed 

 Auxanometers. Combined with clockwork a recording auxanometer 

 may be arranged, so as to give continuous readings of the rapidity of 

 growth of a plant under observation. By these means the influence 

 of external conditions upon growth, such as temperature, light, drought, 

 etc., may be accurately studied. 



