

ARRANGEMENT < >F THE MES< (PHYLL 



hires 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. -'<>). 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 it' the cells were separated by intercellular spaces. Shade 



Fig I'- 



ll,, tion of .t leaf <>f Rhododendron. X"t<- the compact palisade 



which enables the planl t" withstand drying winds. 

 I 1,. .11 of .1 leaf of skunk cabbage, Spathyema, Note the 



i„,.,r!. ed palisadi >>.l ili<- loose arrangement of the cells 



of this plant which lives in m<>i-t, shaded pit 



