IRRITABILITY. 553 



that more experimental evidence must be forthcoming before 

 it is possible to decide between them. 



In conclusion, the general mechanism of the movements 

 of variation may be briefly alluded to. The fundamental 

 fact to be apprehended is this, that all these movements 

 depend upon variations in the turgidity of the cells of the 

 motile organs. When the organ simply shortens, as in the 

 case of the stamens of the Cynarese, it is because the turgidity 

 of all the cells has diminished. When the organ performs an 

 up and down movement, as is the case, for instance, in motile 

 foliage-leaves, it is because the relative turgidity of the upper 

 and lower halves of the pulvinus varies ; when the organ 

 moves up, the lower half of the pulvinus is more turgid than 

 the upper, and conversely, when it moves down, the upper is 

 more turgid than the lower. The details of the mechanism 

 will form the subject of the next lecture. 



BIBLIOGRAPHY. 



Movements of Locomotion : 



Max Schultze; Arch. f. mikroskop. Anat., I, 1865. 



Engelmann; Bot. Zeitg., 1879. 



Nageli; Gattungen einzelliger Algen, 1849. 



Dippel; Beitrage zur Kenntniss der in den Soolwassern von Kreuz- 



nach lebenden Diatomeen, 1870. 

 Borscow; Die Siisswasser-Bacillarien des siidwestlichen Russlands, 



1873. 



Mereschkowsky ; Bot. Zeitg., 1880. 

 Hansgirg; Bot. Zeitg., 1883. 

 Cohn; Arch. f. mikroskop. Anat. Ill, 1867. 

 Nageli; Beitrage z. wiss. Bot., II, 1860. 

 Pfitzer; Unters. iib. d. Bau und Entwickelung d. Bacillariacese, in 



Hanstein's Bot. Abhandlungen, I, 1871. 

 Stahl; Bot. Zeitg., 1880. 



Pulsating Vacuoles : 



Cohn; Nova Acta Acad. Caesar. Leopold., 1854; Beitr. z. Biol. d. 



Pflanzen, II, 1877. 

 Engelmann ; Zur Physiologic d. contractilen Vacuolen der Infusions- 



thiere, 1878, 



