186 STECKBECK— ON COMPARATIVE HISTOLOGY 



Nyctitropic movements; also called night sleep of plants, since the 

 rising and sinking of leaves and leaflets coincides with the nocturnal 

 sleep of animals and the phenomenon was interpreted earlier in this 

 sense. 



Leaf sensitivity as usually understood is almost wholly confined to 

 dicotyledons, although there are several monocotyledons, g>^mnosperms 

 and pteridophytes that exhibit this peculiarity. 



Darwin describes sleep movement of the leaves of one of the water 

 ferns, Marsilea quadrifoliata. Each leaf bears four leaflets, each of 

 which is provided with a well-developed pulvinus. During the late 

 afternoon, the leaflets move upward and fold upon each other in such 

 a manner as to form a vertical packet. "When the leaves sleep, the 

 two terminal leaflets rise up, twist half around and come into contact 

 with one another and are afterwards embraced by the two lower leaflets. " 



Among the monocotyledons, probably the best known example of 

 night turning of leaves is shown by various species of Maranta. During 

 the day the rather large blades of the leaves are in an approximately 

 horizontal position, while at night they stand vertically. 



Amongst dicotyledons, two families stand out prominently as exhib- 

 iting the most specialized types of leaf sensitivity, these the Leguminosae 

 and the Oxalidaceae, two families that in sensitive relations, in struc- 

 tural details, and in distribution show many similarities. Both contain 

 forms that are practically non-sensitive with all transitions from these 

 to the very highly sensitive types. Both have compound leaves in 

 nearly all members, with well-defined primary pulvini at the bases of 

 the petiole, secondary pulvini in some and tertiary pulvini in those with 

 bi-pinnately compound leaves. Both families are essentially tropical 

 in distribution of the sensitive types. 



This investigation is confined to the Leguminosae and the Oxali- 

 daceae, for practically all sensitive plants, as the term is usually applied, 

 belong to these two families. Other sensitives, such as Drosera and 

 Dionaea, are omitted. These have been studied very extensively by 

 Darwin, Macfarlane and others. 



The work was suggested by Professor John M. Macfarlane, of the 

 University of Pennsylvania, to whom I want to express my deep grati- 

 tude for his advice, aid and criticisms. 



Historical Review 

 The phenomenon of leaf sensitivity and leaf movement has proved 

 of botanical interest since the days of the early Greeks. From the time 

 of Pliny, who made studies of such movements, the historical records 



