43 6 



PHYSIOLOGY 



organs. This is very clearly seen in the sporangiophores of Mucor and 

 the rhizoids of Marchantia, as will appear from the graphic representa- 

 tion of the observations (figs. 673, 674). It comes out also in the auto- 

 graphic records of the growth of elongating stems when plotted so 

 as to show the increment during the day and during the night, the 

 temperature and other conditions, of course, being kept as constant 

 as practicable. 



Daily period. — In nature the retardation due to light is doubtless 

 accentuated by the greater evaporation of the daytime; but it is more 



7 

 6 



5 

 4 



3 



2 



t 



mmL 



60 



T 



F° 



8 10 Mm. 2 4 6 8 10 12;/. 2 4 6 8 10 

 Fig. 675. — Curve of daily period (solid line) and of temperature (broken line) : each 

 vertic al interval corresponds to i mm. increment for the growth curve and to 5 F. for the 

 temperature curve; horizontal intervals are hours, the region of close-set lines showing 

 the night. Note rapid growth during the first day, beginning to fall off before the tem- 

 perature falls (probably a transpiration effect) and then rising in the night in spite of 

 falling temperature (partly also a moisture effect). — From data by Spoehr. 



or less compensated by the acceleration due to the rising temperature. 

 Contrariwise, the acceleration upon the coming of darkness and a moister 

 air is partly offset by the retardation due to the lower temperature of the 

 night. Nevertheless, a periodic variation in growth in length, corre- 

 sponding to the day and night, and hence called the daily period, can 

 be traced, unless the fluctuations of temperature are excessive. This 

 means that as certain conditions act antagonistically upon the rate of 

 growth, they may be balanced or one set may overcome the other. The 

 difference between the darkness of night and the light of day is so much 

 greater than the usual differences of temperature and moisture in these 

 hours, that the light effect is likely to be dominant (fig. 675). 



