IRRITABILITY. 



6i 



47. Periodic Movements. — The sun is continually changing 

 its position during the day ; consequently if a leaf remains in a 



Fig. 56. 



Fig. 57. 



Day and night positions of leaflets of Bean. (Detmer.) 

 fixed position it receives the maximum heat and light at one 

 moment only. It is found that leaves not only exhibit move- 

 ments corresponding to the heat and light received, but also 



assumed certain positions 

 to avoid excess or loss of 

 heat. An organ loses or 

 receives the least heat 

 when its long axis is 

 vertical. A great num- 

 ber of these movements 

 have been described as 

 " sleep movements." 

 EXPERIMENT 75. 



SLEEP MOVEMENTS. 



Observe the positions of 

 Sleep position of leaves of Oxalis induced the leaflets of a seedling of 

 by artificial darkness. (Hansen.) Bean or of an Oxalis, grow- 



ing in the sunlight, at 8 a.m., i p.m., and 6 p.m. Determine whether 

 these positions are due to Hght or heat by use of the dark chamber. 

 (Figs. 56 and 57.) 



