128 THE CARBOHYDRATES 



Function. 



Mucilage, when it is a definitely secreted product or of a 

 definite and constant occurrence in a plant, may perform several 

 functions, but how far these are primary functions cannot yet 

 be stated. 



When it occurs in tubers, as in the Orchidaceae, mucilage 

 is generally looked upon as a reserve food-material ; it may 

 serve as a check against too rapid transpiration, especially 

 when produced in connexion with developing organs, such as 

 vegetative buds, young leaves, in the epidermis of mature 

 leaves, the sporangia of Cryptogams, etc. ; in the case of 

 aquatic plants, such as Algae, the hibernaculae of Myrio- 

 pkyllum, etc., its presence may prevent a too rapid diffusion ; 

 the calcareous incrustation of certain Algae, e.g., Neomeris 

 dumetosa, is dependent on the presence of mucilage ; mucilage 

 provides a water-storage mechanism in plants subjected to 

 xerophytic conditions, e.g., Cassia obovata, Malva parviflora, 

 Theobroma cacao, and Pterocarpus saxatilis ; finally, it may be 

 an important aid in connexion with seed-dispersal and ger- 

 mination, as in some species of Salvia and Lepidium. 



Related to the gums and mucilages are the substances 

 known as galactosanes occurring in the seeds of Leguminosae 

 (Lupinus> Medicago, etc.) ; wood gum or xylane, occurring in 

 wood, etc. etc. These substances have already been mentioned 

 in connexion with the sugars which they give rise to on 

 hydrolysis. 



PECTIC BODIES. 



Many succulent fruits, such as pears, apples, gooseberries, 

 and currants, and also fleshy roots, such as carrots, beetroots, 

 etc., contain, together with the cellulose in the cell walls of 

 parenchymatous elements, a substance which is soluble in 

 water, but whose aqueous solution gelatinizes on the addition 

 of alcohol. This substance, which is also probably the cause of 

 concentrated aqueous extracts of fruit gelatinizing, is known 

 as pectin. 



According to Fremy,* the hardness of unripe fruit is due 



* Fr6my : " J. Pharm. et Chim.," 1840, 26, 368. 



