PLANT MANUFACTURE OF FOOD 133 



and is so resistant to enzyme action that few organisms can 

 digest it except bacteria, fungi and some protozoa. 



Numerous other carbohydrates, of lesser importance and less 

 well known, are derived by plants from glucose. Dextrins are 

 compounds intermediate between starch and maltose; they are 

 gummy substances typified by those used in making library 

 paste and the sticky back to postage stamps. Various gums, 

 formed by the breakdown of cellulose, are used to protect stems 

 and seeds from excessive loss of water and low temperatures. A 

 familiar substance of this type is gum arabic. Related to the 

 gums are the various mucilaginous substances produced com- 

 monly by numerous kinds of plants; these may act as reserve 

 foods, may prevent excessive transpiration, or may aid in seed 

 dispersal. The roots and flowers of the mallows produce the 

 mucilaginous substance which forms the basis of marshmallow. 

 In the mistletoe, the mucilage aids in seed dispersal; when a bird 

 has eaten a berry of the mistletoe, the seeds stick to his beak. In 

 an attempt to rub them off", the seeds are stuck firmly to a twig 

 or a branch which will soon be the host to a germinating mistle- 

 toe parasite. Other glucose-derivatives include the pectins and 

 pectin compounds produced in the cell walls of fruits; these are 

 utilized by housewives since it is the pectin which makes the 

 fruit juice jell. A commercial jelling compound is made from 

 the pectins of apples. 



Fats and Oils 



Sugars and starches can be transformed by both plants and 

 animals into compounds consisting of the same three elements, 

 but these compounds — known as fats and oils — have greater 

 amounts of carbon and hydrogen, less oxygen, than are found in 

 carbohydrates. For this reason they produce greater amounts of 

 energy when oxidized. A fat will yield two and a quarter times 

 as much energy as a carbohydrate of the same volume. Animals 

 store and use fats as sources of energy more commonly than 

 plants, which utilize them chiefly as reserve foods in organs where 

 there is a premium on space. Thus we find that most of the plants 

 producing fats or oils use them as reserve foods in seeds in order 

 to get as much energy as possible into a small space. 



