AND IRRITABILITY OF SENSITIVE PLANTS 207 



latutn are all upright growers, and are therefore exposed in their leaflets 

 to the full effects of night cold and heat radiation from the tissues, and 

 I believe that this may largely explain why in evolutionary development 

 they have become much superior to Desmodium rotundifolium, whose 

 long, sucker-like shoots run along the ground, and give off leaflets that 

 nestle among the surrounding herbage." 



D. rotundifolium shows night sleep in its leaflets and also parahelio- 

 tropic movement. The leaflets are slightly sensitive to mechanical 

 and thermal stimuli. Both of these forms of stimulation cause a drop 

 of the leaflets through a relatively small angle but only after a long 

 latent period. 



Leaflets. Crystals are present in the endodermis of the orbicular 

 leaflets. The crystals, which are arranged in lines with occasionally 

 a small gap in the crystal cell continuity, are small prisms, mostly of 

 the six-angled t>'pe. The variation in shape and size of crystals is most 

 marked. The average size for a number of the cr}^stals is 9.5 microns 

 long and 5.7 microns wide; the average dimensions of the crystal cells 

 are 19.5 microns long and 14.2 microns wide. 



Pulvini. In both primary and secondary pulvini irregular crystals 

 are scattered throughout the cortex. These vary in shape from forms 

 that approach the conglomerate type to others that are quadrangular, 

 and relatively few that are hexagonal, but more elongated and not as 

 wide as the hexagonal forms in the leaflets. 



Petiole. The crystals are of the same kind and show similar distri- 

 bution to those in the leaflets. 



Desmodium Dillenii 



This species is very common in the eastern half of the United States. 

 From the few experiments made, the writer considers this plant to be 

 close in its sensitive action and relation to D. rotundifolium, but slightly 

 more sensitive, in that when the leaflets are stimulated mechanically, they 

 drop after a shorter latent period than in the species previously described. 

 The angle through which the leaflets drop is also greater than in the 

 former species. 



Leaflets. The crystal distribution in the leaflets of this species 

 resembles rather closely that of Desmodium rotundifolium, but the crystal 

 lines along the veins show fewer gaps. The crystals are larger rela- 

 tively to the size of the cells in which they are contained. The hexa- 



