142 PLANT PHYSIOLOGY 



Gum tragacanth comes from another member of the legume 

 family (Astragalus) and is used for very much the same purposes as 

 gum arabic. When hydrolyzed it yields arabinose, xylose, galac- 

 tose, and geddic acid, which is an isomer of arabic. It is obtained 

 by wounding the stem, under which conditions the gum results 

 from the metamorphosis of the medullary rays. 



Cherry gum or cerasin is often found on the stems of the cherry, 

 plum, and peach (Prunus). When hydrolyzed it yields almost 

 entirely arabinose. Closely related to it are the wound gums which 

 seem to be secreted by living cells of the wood in proximity to a 

 cut or wound. Their function is not well known. In composition 

 they are related to both gums and cellulose. 



Mucilages. — The mucilages differ from the gums in that they 

 do not dissolve in water, and from the pectins in that they do not 

 gelatinize. They swell up in water and produce a slimy colloidal 

 solution. They occur in many places throughout the plant king- 

 dom: — in the roots and flowers of mallows (from which marsh- 

 mallow comes); in onion bulbs; in the succulent leaves and stems 

 of the Agave, Yucca, and cacti; in the fruit of the banana; in the 

 seed of the mistletoe; in mucilage-secreting hairs; and in the 

 walls of many of the algae like Spirogyra, Nostoc, and Lami- 

 naria. 



The chemical composition of the mucilages is not exactly known, 

 but when hydrolyzed they yield pentoses and hexoses, chief of 

 which are xylose, glucose, and galactose. Wiesner considers them 

 to be formed from the action of an enzyme on cellulose. 



Although classified as structural materials, the mucilages seem 

 to have various uses in the plant. In aquatic plants they may aid 

 to prevent too rapid outward diffusion of substances in the cell. 

 When found on the epidermis they probably check transpiration, 

 and in desert plants Spoehr and others have shown that they play 

 an important role in retaining water within the plant, where, due 

 to their water-holding capacity, they serve as an integral part of 

 the storage mechanism. When found in tubers, as in the orchids, 

 or in fruits, it is assumed that they act as reserve foods, although 

 whether such difficultly hydrolyzable compounds can be used for 

 food is doubtful. In connection with seeds such as Salvia and 

 mistletoe, the mucilage aids in seed dispersal. The mucilaginous 

 seeds of the mistletoe stick to the beaks of birds which have eaten 

 the fruit and, when wiped off, remain upon the limb of the tree. 



