216 STECKBECK— ON COMPARATIVE HISTOLOGY 



account of the very striking sensitive movement of the leaves the plant 

 has long been a fa\orite object for the study of irritable responses. 

 The relative ease of growing this plant in greenhouses in different 

 climates has been another helpful factor in making it a world wide 

 favorite for laboratory purposes. The leaves are highly sensitive 

 to nyctitropic, paraheliotropic, mechanical, chemical and other forms 

 of stimulation. The direction of movement and the position after 

 contraction is the same for the different kinds of stimuli just given. 

 That is in all cases of stimulation the secondary leaflets move upward 

 and forward, the midribs converge and the whole leaf drops. Such 

 complete closure, to be sure, takes place only under optimum environ- 

 ment. 



Cotyledons. The cotyledons of Mimosa pudica show sensitive 

 movement when they are quite young — 3 to 8 days old. When older 

 they become yellowish and lose all sensitivity. Darwin (11, p. 127) 

 considers the cotyledons of this plant to be very feebly sensitive. Mac- 

 farlane (31, p. 203) states that the cotyledons are markedly sensitive. 

 "The cotyledons of the Sensitive Plant are most active during the 

 period that their activity is of greatest benefit, viz. in the very young 

 state (seedlings 2 to 10 days old), since their great function is to protect 

 the first leaves and the growing bud. When the latter have pushed 

 out above the cotyledonary tips, the protective function has ceased, 

 and their irritable movements are greatly lost. " Two to three stimuli 

 are required to cause the cotyledons to rise through an angle of 80 

 degrees. The movement is rather slow, resembling the movement of 

 the leaves of less sensitive plants, like some of the Desmodiums. 



Large conglomerate crystals are scattered through the mesophyll 

 of the cotyledons. No prismatic crystals are present. The pulvini 

 of the cotyledons are devoid of crystals. 



Leaflets. The crystals are mostly of the hexagonal type, with 

 relatively few of the quadrangular forms. The crystal cells are con- 

 tinuous along the veins, forming close sheaths on the ventral side of 

 the vascular bundles. The crystals are large, usually filling % to ^ 

 of the cell cavities. Very often the crystals touch the dividing cell 

 walls. 



Pulvini. No crystals are present in the pulvini. 



Petiole, Stem and Root. A continuous system of crystal cells, which 

 form sheaths round the bundles, is present in the petioles and in the 

 stem of Mimosa pudica, with breaks only in the continuity in the pul- 

 vini. The regular crystals are also present in the sensitive lanceolate 



