212 STECKBECK— ON COMPARATIVE HISTOLOGY 



Pulvini. There are relatively few crystals present in the pulvini. 

 These are all of the regular type found in the leaflets, and are located 

 in the endodermal region, immediately around the bundles. 



Petiole. The crystals in the petiole and midrib are so abundant 

 as to form a crystal sheath that covers the vascular bundles, the main 

 crystal development being found on the ventral side of the petiole. The 

 crystals are of the regular type noted in the leaflets. Branches., of 

 crystal cells extend into the bases of the petiolar glands and can be 

 traced to a distance representing i^ the height of each gland. The 

 crystal cells do not have the same regular continuity that they have 

 in the crystal lines of the petiole, but are more broken. 



Cassia nictitans 



This species, the Wild Sensitive-Plant of the eastern United States, 

 shows close similarities, in its structural and histological details and in 

 its irritability, to Cassia chamaecrista. Each pinnately compound leaf 

 bears 8 to 20 pairs of leaflets. Britton (6, p. 529) states the number 

 to be 12-44. The larger number probably applies to plants growing 

 further south. On the upper surface of the petiole, near the basal 

 pair of leaflets, a slightly-stalked gland is borne. The gland in this 

 species, as in the last, is an irritable center. 



The movements of the leaves under nyctitropic and paraheliotropic 

 stimulation are like those described for Cassia chamaecrista, except that 

 this species is slightly more sensitive, not only to light stimulation, but 

 also to mechanical and chemical stimuh. 



The illustration (Plate LVIII, Fig. 1) shows the expanded leaves of 

 young plants grown in the green houses. In night sleep (Plate LVIII, 

 Fig. 2) the leaflets are folded upward, back to back, and the leaves have 

 dropped from the day position. 



The upward movement of the leaflets in paraheliotropic stimulation 

 begins at a slightly lower temperature than for C. chamaecrista. Here 

 the movement begins at 24° to 26° C. in bright sunlight. As the tem- 

 perature rises, the movement continues until a maximum closure is 

 reached at about 32° C. 



Under optimum environal conditions two mechanical stimuh cause 

 a maximum closure of the leaflets and a drop of the entire leaf. After 

 one stimulus has been applied there follows a latent period of 3^ seconds, 

 and then a closure of the leaflets through an angle of 40 degrees. During 

 the upward and slightly forward movement of the leaflets, the midrib 

 of the leaf stimulated drops 35° to 40°. If a second stimulus is then 



