MORPHOLOGY OF HIGHER PLANTS. 



175 



product of the protoplasm, or it may be a disorganization product 

 of some of the carbohydrates of the cell-contents. When it arises 

 through modification of the wall it is spoken of as " membrane 

 mucilage" (Fig. 99), and owes its origin to several causes: 

 either to a secondary thickening of or an addition to the cell wall, 

 or a metamorphosis of it, at least in part. In the latter case it 



Fig. 98. Cell-content mucilage in sub-epidermal cells of Viola tricolor: A, surface 

 view of portion of petal, the cells without marking being above the mucilage-cells; B, 

 transverse section of leaf showing epidermal cell and sub-epidermal mucilage-cell beneath 

 it; C, surface view of a sub-epidermal mucilage-cell. 



may arise either as a disorganization product of the primary wall, 

 or of the subsequent lamellae making up the walls of the cells of 

 the pith, medullary rays, parenchyma and other tissues, as in 

 Astragalus giuninifcr (Fig. 274), or it may arise as an inter- 

 cellular substance. 



The following is a classification of some plants, based upon the 

 origin of the mucilage which they contain : 



