226 STECKBECK— ON COMPARATIVE HISTOLOGY 



4. The paraheliotropic response probably next succeeded, v/hile 

 response to light stimuli, and to mechanical stimuli next became pro- 

 nounced. 



5. Under experimentation, equally marked responses to chemical, 

 thermal and electrical stimuli are observed. 



6. In relation to previous studies and discussions on propagation 

 of stimuli, the writer advances the view that this phenomenon is centered 

 in the endodermis. The cells of this layer contain a greater or less 

 number of crystals of oxalate of lime, which in number and in progressive 

 perfection of development show transition from less sensitive to the 

 most sensitive types. 



7. Amongst the less sensitive plants, the crystals are often con- 

 glomerates, irregularly scattered outside the endodermis, or in discon- 

 tinuous lines along the endodermis and surrounding the bundle. 



8. With increasing sensitivity and advance toward tropical conditions 

 the crystals show, alike in the Leguminosae and the Oxalidaceae, similar 

 progressive advances in abundance of crystals, regularity of shape in 

 these, and restriction of them to the endodermis. 



9. The cHmax in this process is seen in Mimosa pudica and M. 

 Spegazzini amongst the Leguminosae, and in Biophytum sensitivum 

 and B. dendroides in the Oxalidaceae. These represent also the most 

 sensitive plants of the two families. 



10. In histological distribution the crystals and crystal cells of the 

 endodermis become in the most sensitive species continuous from the 

 leaf margins and even in M. Spegazzini from the bases of the hairs, 

 along all of the veins of the leaflets, the petiole and the stem. But in 

 the irri to-contractile pulvinus regions, that receive and respond to stimuli, 

 the crystals are absent. In less sensitive pulvini, however, conglomerate 

 and transition crystals toward the most perfect type are present. 



11. Each crystal cell in the higher types contains a rhombohedral 

 crystal that is traversed across its middle by a cleavage plane, that is 

 first affected by solvents such as concentrated hydrochloric and nitric 

 acids. 



12. Surrounding each crystal is a protoplasmic sac that stains 

 deeply with protoplasmic stains, while from the sac intercellular uniting 

 threads seem to pass through the common membranes between the 

 adjacent cells, so as to form continuous protoplasmic connections 

 throughout the endodermal tissue. 



13. The writer would view the crystals and the continuous proto- 

 plasmic investments and connections between these as the special 

 conducting lines for the passage of stimuli. 



