154 



Hydration and Growth. 



stationary at midday, with a stem temperatiire of 26° to 29° C. Actual 

 shrinkage now began and continued through the afternoon, but all action 

 ceased at night. On the following day swelling or increase in thickness began 

 at 8 a. m. at a temperature of 12.5 °C., but continued for an hour only. The 

 leaves were beginning to flag at lO'^SO™ a. m., as the plants had not been 

 watered, and shrinkage was in progress before noon at a temperature of 

 23.5° C." (See fig. 38.) 



Fig. 38. — Detail of arrangement foi; recording variations in thickneas of stem of Helianthus. 

 A, stem; B and C, parts of clamp holding stem rigidly in place; D, support; E, cork bearing 

 of short lever of auxograph; F, pen arm of auzograph, and G, rack-and-pinion column of 

 auxograph. 



A second preparation was set up on February 10, in which the 

 variations in thickness were taken at a place but 6 cm. from the tip of 

 the stem. The adjustment had been completed by night and an in- 

 crease in thickness of 0.38 mm. took place in 12 hours at temperatures 

 between 24° and 18° C. Constant readjustment was necessary to 

 obtain reliable measiu-ements, and on February 12 another record was 

 obtained, at which time an enlargement of 0.4 mm. was recorded 

 between 11 a. m. and 2''30'" p. m. at temperatures of 22° to 25° C. 

 After this time a shght shrinkage occurred, although the plant was so 

 well supplied with water as to show no indications of wilting. Here, 

 as in the leaves of Mesembryanthemum, elongations may be taking 

 place at a lessened rate in the extreme terminal part of a stem in which 

 the hydration capacity is kept continuously higher than in older inter- 

 nodes, while at the same moment an actual decrease in thickness may 

 be taking place within a few centimeters of the elongating active zone. 

 This shrinkage may ensue in a section of the stem which has not lost 

 the capacity for elongation altogether, so that its daily record shows a 

 period of elongation at a moderate rate during a part of the day, then 

 a cessation due to the depletion of the water-balance.^ It is obvious 

 that the action in question is one which may be responsible for many 

 mistaken generaUzations bearing upon cessation of growth and effect 

 of temperature on growth (fig. 39). Attempts at interpretation of the 



'See Brown and Trelease. Alternate shrinkage and elongation of growing stems of Cestrum 

 nocturnum. Philipp. Jour, of Science, 13: No. 6, 333. 1918. 



