AND IRRITABILITY OF SENSITIVE PLANTS 20S 



of stimulation. It is quite probable that this outlying species of a 

 genus, that is tropical and sub-tropical in its distribution, has lost its 

 sensitivity. The protection afforded by a closure of the leaflets is not 

 necessary in this plant. The small leaflets are sufficiently protected 

 against a too rapid radiation of heat by the well developed cuticle. 

 Leaflets. Broken lines of cr>'stals abound along the veins of the leaflet. 

 One to six or eight lines of cr>'stals are present, according to the size 

 of the vein. In shape the crystals are quite regular, mostly four-angled, 

 and relatively few six-angled. The size of the crystals is extremely 

 various, from the smallest ones less than 3 microns long, to the largest 

 ones, 15 microns long. The cr>'stals show no definite polarity of 

 arrangement with respect to the direction of the vascular bundles. In 

 many cells the longer axis of the crystal is at right angles to the greatest 

 length of the cell containing the cr\'stal. 



Pulvini. In the secondary pulvini, as well as in the primary pulvi- 

 nus of each leaf, are found the same types of crystals as in the leaflets, 

 but these are more scattered in their arrangement. There are few 

 conglomerate cr>'stals present in the cortex of the pulvini. 



Petiole. The crystals of the leaflets, the pulvini, the midrib and the 

 petiole of the honey locust are very similar throughout in shape and 

 arrangement, but with a greater number of crystals in the leaflets and 

 in the petiole than in the pulvini. The cr>-stals are imbedded in a 

 proto-plasmic sac or envelope which is not as dense as the envelopes in 

 the more sensitive plants are, as is indicated by the rather hght stam 

 of the enveloping sheaths when protoplasmic stains are applied. 



Gymnocladus dioica 



Gymnocladus is a small genus, including two species, G. chinensis 

 of eastern Asia, and G. dioica (the Kentucky Coffee Tree) of eastern 

 North America, from southern Canada to Oklahoma. The bi-pinnately 

 compound leaves of the latter species show night sleep very strikingly. 

 When observed at night the leaflets are in a position that is almost 

 vertical, having moved downward. The movement and the position 

 of the leaflets at night are very similar to those of Robinia. The dis- 

 tribution of the two plants is almost exactly identical with a slightly 

 wider range for the Kentucky Coffee Tree. The paraheliotropic response 

 is not as marked as it is for the leaflets of Robinia. In the latter the 

 upward movement begins when the temperature reaches 24° to 27° C. 

 and when there is bright sunlight, but Gymnocladus requires a higher 



