macdougal: mechanism and conditions of growth IQ 



retardation effect became more accentuated with the development 

 of the joint until an actual cessation of growth occurred on the 

 night of May 9 and 10. The retarding conditions from this 

 time became accentuated so that on May 11 actual shortening 

 was displayed, and the remainder of the period of observation 

 was made up alternating daylight-elongating phases and nocturnal- 

 shortening phases (fig, 4). 



Fig. 4. Auxographic tracing of growth and variations in length of Opuntia Blakeana, 

 No 13, for the period month beginning May 6, and ending June 5, 1916. Elongation 

 IS denoted by upward movement of the pen and the actual change in length is multi- 

 plied 9f times. 



At the time of maximum rate of elongation the daily increase 

 was equivalent to nearly i mm. In tHe period following, when 

 the daily increase ceasing in mid-afternoon was followed by a 

 decrease in length, the elongation amounted to .5 mm. daily, and 

 would be followed by a shortening of .3 mm., leaving a net total 

 growth daily of .2 mm. This net accretion diminishes with the 

 approaching maturity of the joint. This last-named record was 

 taken during the high temperatures of May. 



The three types of change in volume illustrated by these mea- 

 surements agree in that accelerating elongation coincides with the 

 duration and increasing intensity of illumination in the forenoon 

 and that the continued rise in temperature (within the tonic range) 

 and of the illumination is accompanied by a retardation or a 

 shrinkage in the afternoon. 



If we consider the known possibilities in the way of change of 

 volume attention would naturally be turned first to water-loss. 



