286 THE MOVEMENTS OF WATER 



exposed to strong diffuse light. In certain other flowers (Frilillaria, 

 Ilcllebonis] the secretory activity is developed in darkness, as is also 

 the case in the foliar nectaries of Prnnns lauroccrasus and Cassia neglecta. 

 A special investigation is therefore necessary in each case to determine 

 whether light influences the secretion of nectar directly or only indirectly. 



The functional activity of the nectaries can only be developed at 

 a certain temperature, but when it has once commenced, certain nectaries 

 may continue to excrete at a temperature below that necessary for the in- 

 ception of their secretory activity. Wilson observed that the foliar nectaries 

 of Primus laurocerasi/s continued to excrete nectar at from J to 5 C. 



Honey-dew, &c. It is not necessary to discuss all the cases in which 

 sugar may be normally or pathologically excreted. The formation of 

 honey-dew is a pathological phenomenon, and usually, but not always, as 

 Biisgen supposed, aphides are the exciting cause. Bonnier has shown 

 that honey-dew may appear when no aphides are present, and it then 

 frequently oozes out through the stomata l . The formation of honey-dew 

 is favoured when cool nights follow warm and dry days ; Bonnier was 

 able indeed to cause a production of honey-dew by keeping in a saturated 

 atmosphere branches which had been immersed in water. The honey-dew 

 may be formed in such abundance, especially in damp weather, that it may 

 drip from the trees, as is the case in the so-called rain-trees of the tropics 

 (Caesalpinia pluviosa,) Calliandra 



1 Bonnier, Rev. gen. d. Bot., 1896, T. vrn, p. 22 ; Biisgen, Per Honigthau, 1891, and here the 

 related literature will also be found. 



2 Cf. Unger, Sitzungsb. d. \Yien. Akad., 1857, Bd. xxv, p. 450; Boussingault, Agron., Chim. 

 agric., &c., 1874, T. v, p. 33 ; Mussel, Bot. Jahresb., 1879, p. 222 ; Biisgen, 1. c., p. 26 ; Dyer, Bot. 

 Jahresb.. 1878, p. 326. 



