Some Hydration Reactions and Growth. 



147 



25 



manner described, and a portion of the record is given in figure 35. As 

 may be seen, the period of shrinkage, which begins about the same time 

 in the older leaf, continues for a shorter period, on some days not more 

 than 2 hours, and enlargement sets in in mid- 

 afternoon. The thickness on each successive 

 morning was greater than at the same time on 

 the preceding day, demonstrating that actual 

 growth was in progress. 



Two series of measurements were now un- 

 dertaken to secure new records of the elonga- 

 tion of leaves which had reached about half 

 the final length and of others still younger. 

 Such a pair of young leaves, with their surfaces 

 still appressed in an erect position, were 

 brought into bearing on an auxograph lever in 

 a sunny place in a glass-house. The length of 

 the exposed portion was about 25 mm. and 

 their thickness was not over 0.5 nam. at the 

 beginning of the tests. Here, as in previous 

 preparations, it was found that whatever the 

 causes of the stoppage of growth and of 

 shrinkage might be, they were not effective in 

 producing an actual cessation of elongation, 

 which in these young leaves continued through- 

 out the 24 hours of the day variously respon- 

 sive to alterations in temperature (fig. 36).^ 



Another pair which were about to spread by the growth of the bud 

 ensheathed between their bases were attached to the auxograph and 

 set in a place where they would be shielded from the direct rays of the 

 sun. Preparations of all three stages increased in length and thickness 



5 



15 

 25 

 35 



45 

 55 

 65 



Fio. 35 — Auxographic record of thickness of pair of leaves about half mature size. X45. A 

 daily shrinkage between 8 a. m. and midday occurred, which amounted to an increasing pro- 

 portion of the increase taking place during the remainder of the day. 



during the entire night, the increase in thickness being very rapid 

 diu'ing the first half of the night and slowing down to a very low rate 



^MacDougal and Spoehr. Growth and imbibition. Proc. Amer. Phil. Soc, 56: 310. 1917. 



Fig. 34. — Upper part of fig- 

 ure is an auxographic rec- 

 ord of variations in thick- 

 ness of leaf of Mesembry- 

 anthemum on a cloudy 

 day with but little change 

 in temperature, as taken 

 by a mercurial thermom- 

 eter from a similar leaf. 

 Upward course of line de- 

 notes increase. X45. 



Lower figure is an auxo- 

 graphic tracing of same 

 leaf on sunny day with 

 temperatures of 16° to 25° 

 C. Shrinkage occurred 

 during the entire day- 

 light period. X45. 



