36 ARRANGEMENT OF THE MESOPHYLL 



tures the skin while the distended basal portion contracts and 

 forces out some of the fluid. The burning sensation or sting 

 that immediately follows the puncture of the skin is caused by 

 the injection of formic acid while a variety of other poisons pro- 

 duce the subsequent irritations. These poisons are so powerful 

 in some of the East Indian nettles as to produce serious results, 

 even tetanus or lock-jaw. 



17. Significance of Certain Internal Leaf Structures. The 

 changes or modifications produced in the chlorenchyma by vari- 

 ous stimuli are quite as striking as in the case of the epidermis. 

 The palisade cells are an excellent illustration of this point. 

 These cells are developed as a result of the stimulating influence 

 of light (Fig. 26). Leaves growing in the deep shade show 

 little indication of palisade structure. The compact arrange- 

 ment of these cells prevents to a large degree the access of the 

 hot, dry air and consequently loss of water is less than would be 

 the case if the cells were separated by intercellular spaces. Shade 



FIG. 26. FIG. 27. 



FIG. 26. Section of a leaf of Rhododendron. Note the compact palisade 

 tissue which enables the plant to withstand drying winds. 



FIG. 27. Section of a leaf of skunk cabbage, Spathyema. Note the 

 poorly developed palisade tissue and the loose arrangement of the cells 

 of this plant which lives in moist, shaded places. 



