MORPHOLOGY OF HIGHER i'LANTS. 



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: 

 cither 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. Citrus vulgaris. Longitudinal section of a young fresh fruit showing a lysig- 

 enous oil canal or duct. Se, oil; Zs, cell sap; PI, cells in which the walls have been dis- 

 solved; f, thin-walled cells; D, thick-walled cells; K, nucleus; Chr, chromoplasts; o, crystals 

 of calcium oxalate; e, epidermis. After Meyer. 



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

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

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

 Astragalus guminifcr (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 (hey contain : 



