GROWTH AND MOVEMENT 



121 



which arc 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 ir> 

 different individuals, and according to external circumstances. In 



1416 18 20 22 24 26 28 30 32 34 36 3840 



Temp. 



FIG. 84 bis. 



Chart showing the relations of growth (in mm.) 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'Arcy Thomson, based on observational data of Sachs.) 



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

 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. 



