218 A TEXT-BOOK OF BOTANY. 



through chemical affinities with the fatty material of which it is 

 composed. Others are inclined to think of protoplasm as an 

 emulsion of proteins and * lipoids.' Loeb and v. Knaff Lenz find 

 that sea-urchin eggs are liable to undergo cytolysis under the 

 action of any process, chemical or physical, that causes the cell 

 fats to become more fluid." (Consult J. B. Leathes, " The 

 Fats.") 



Mucilages and Gums. By the terms mucilages and gums are 

 meant those substances which are soluble in water, or swell very 

 perceptibly in it, and which, upon the addition of alcohol, are 

 precipitated in the form of a more or less amorphous or granular 

 mass. Mucilage originates in the plant as a cell-content, or as a 

 modification of the wall. In the former case it arises as a product 

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

 of the carbohydrates. When it arises through modification of 

 the wall it is spoken of as " membrane mucilage," 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 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 medullary rays, parenchyma, and other 

 tissues, as in Astragalus gummifer (Fig. 118), or it may arise 

 as an intercellular substance. 



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

 the origin of the mucilage: 



I. Mucilage in the form of a cell-content is of infrequent 

 occurrence in plants. It is usually present in the cells containing 

 raphides, especially in the Monocotyledons. Its origin and de- 

 velopment may be easily followed in the tubers of a number of 

 Orchids, especially those yielding salep. The mucilage arises very 

 early in the development of the cells surrounding the crystal- 

 groups, and continues to be formed as the crystals grow in size, 

 the protoplasm and nucleus being reduced to a very thin layer 

 which lie next to the cell-wall. The mucilage of salep is colored 

 yellowish with iodine and sulphuric acid, or a yellowish-red or 

 rose-red with aqueous eosin solution, and a carmine-red with an 

 aqueous solution of Congo red. The cells containing mucilage are 

 easily differentiated from the surrounding cells by the use of 



