MODIFICATION OF THE ORGANS OF PLANTS 89 



are derived from the modifications of various parts 

 of the plant, either the roots, stems, leaves, or 

 even of the floral stems. 



This modification is always accompanied by some 

 degeneration ; the thorn, which is quite hard, ex- 



iG.51.Mamillaria elephantident (after Lemaire? The figure is taken from 

 Goebel, Pflanzenbiologische Schilderungen, vol. i., p. 71). 



cepting at the point, is made up of thick-walled 

 cells from which the protoplasm has disappeared. 



Leaf thorns arise from the modification of various 

 parts of the leaves either of the stipules (as in 

 Caragana), the petioles (id.) (fig. 50), or of the 



