On the Movements of Petals 7 



flowering, etc. ; and, vice versa, sleeping flowers could be made 

 ephemeral by increased heat and dryness, or when the whole 

 flow of sap toward the flower was hindered. In his opinion, 

 turgescence and heat caused opening and closing by the unequal 

 dilatation of the faces; without heat there was no dilatation, 

 without turgescence no elasticity. The internal face, on account 

 of its position, was less exposed to the action of the air and di- 

 lated more than the outer, causing opening. Prolonged and 

 abundant transpiration diminished turgescence, and sleep oc- 

 curred; then the internal face was shortened and the outside, 

 became plane and convex. 



De Candolle, a contemporary of Rover, has been stated by the 

 latter as having emphasized light as the factor causing opening 

 and closing movements. 



Light and moisture, and a certain law of periodicity, were 

 stated by Balfour (1875) to be the cause of sleep movements of 

 flowers, periodicity (or habit) being given almost first place. 



Darwin (1881), as we should expect, held to the latter view, 

 saying that movement was a quality inherited by both plants and 

 animals. He agreed with Pfeffer that nyctitropic movements of 

 flowers are caused by unequal growth of the two sides of the 

 petals due to temperature changes. 



Gustav Zacher (1881) ascribed floral movement of Lotus or- 

 nithopodiodcs to light, but more especially to the variability of 

 the amount of water in the water vessels. 



Pfeffer reports Hofmeister to have said that temperature 

 changes caused movements in garden tulips. 



According to Sachs (1882 and 1887) light was the all-impor- 

 tant factor; temperature and humidity were given a secondary 

 place, it being only occasionally, as in Tulipa and Crocus, that 

 they were the important factors. However, Pfeffer's experi- 

 ments (1876) were mentioned by him, and he certainly consid- 

 ered them valuable. Pfeffer's experiments on Tulipa, Crocus, 

 Adonis vernatus, Ornithogalum wnhellatum, and Cochicum au- 

 tumnalc make heat the most important element. He considers 

 opening and closing mere growth movements : heat and light, as 

 they diminish at night, cause the outer surface to grow faster 



197 



