70 CHEMISTRY OF PLANT LIFE 



MUCILAGES 



These are characterized by forming slimy masses when moist- 

 ened with water. They are secreted by hairs on the skin of many 

 plants, so that the external walls of the leaves, fruit, and seeds 

 are often mucilaginous when damp. This is particularly true of 

 aquatic plants. The chemical composition of the mucilages is 

 unknown. When hydrolyzed, they yield arabinose and a hexose; 

 the latter is sometimes galactose and sometimes mannose. 



When present on the surface of plant tissues, the mucilages 

 probably serve to prevent the too rapid diffusion of materials 

 through the skin, in the case of the aquatic plants, and too rapid 

 transpiration, in the case of young vegetative tissues or in other 

 plants when growing under extremely dry conditions. When 

 found in tubers, or other storage organs, it has been supposed that 

 they may serve as reserve food materials, but it seems that such 

 difficultly hydrolyzable compounds as these can hardly function 

 as normal reserve foods. 



PECTINS 



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

 etc., and many fleshy roots of vegetables, such as carrots, parsnips, 

 etc., contain substances known as pectins. These are readily 

 soluble in water, and when dissolved in concentrated solutions in 

 hot water, they set into " jellies " when the solution is cooled. 

 These jellies carry with them the soluble sugars and flavors which 

 are present in the fruits, and constitute a familiar article of diet. 



There are undoubtedly several different modifications of 

 the pectins, to which the names " meta-pectin," " para-pectin," 

 " pectic acid," " meta-pectic acid," and " para-pectic acid," 

 have been applied. These all seem to be products of hydrolysis 

 of a mother substance known as " pectose," which constitutes the 

 middle lamella of unripe fruit, etc. As the fruit ripens, the pectose 

 is hydrolyzed into the various semi-acid, or acid, bodies mentioned 

 above. The intermediate products of the hydrolysis are the 

 pectins, which swell up in water and readily form jellies; while the 

 final meta-pectic acid is easily soluble in water and resembles 

 the true gums in its properties. When the middle lamella reaches 

 the pectic acid stage, the fruit becomes soft and "mushy" in 

 texture. 



