THE CARBOHYDRATES 143 



Here they are in the proper position for germination, the mucilage 

 also holding sufficient water to start development. 



Pectins. — In the cell wall of succulent acid fruits such as to- 

 matoes, currants, gooseberries, strawberries, apples, peaches, and 

 cherries (as well as in the petioles of rhubarb and the fleshy roots 

 of carrots, beets, and turnips) is a carbohydrate called pectose, 

 protopectin, or pectinogen, which, according to Fremy (1840), 

 causes the hardness of the unripe fruit. As maturation proceeds, 

 this pectose undergoes a series of hydrolytic changes, which ulti- 

 mately result in the formation of pectic acid. Among the prin- 

 cipal intermediate products are the pectins, which (like pectic acid) 

 are insoluble in water, especially if acids are present, and readily 

 form gels in the presence of calcium salts. The jelling of fruit juices 

 is due to the presence of these pectins and pectic acid. Also in 

 fruits which discharge the seeds to some distance when ripe, e. g., 

 Oxalis, Impatiens, Dorstenia, and Lathrsea, the fruit walls have 

 been found to be rich in pectin (Overbeck, 1924), the swelling of 

 which probably aids in this dehiscence. 



The composition of pectins is not well known. Among the 

 sugars finally produced by hydrolysis are arabinose and galactose, 

 while the presence of a "uronic" acid shows the relation of the 

 pectins to the gums. The pectoses have been of prime interest to 

 the plant pathologist, since many of the worst fungal parasites 

 gain entrance to the cell by dissolving the middle lamella, which 

 is made of either pectose or pectocellulose. This pectose (which 

 is probably either a calcium salt of pectic acid, an insoluble ester 

 of that acid, or a compound of a pentosan and a pectin) may 

 serve as food for the fungus; but more frequently the food is 

 derived from the contents of the cells themselves. 



The enzymes which play a part in these various processes are 

 somewhat confused in the literature. The enzyme which dissolves 

 the middle lamella and breaks it down to pectins is called pectosin- 

 ase or protopedinase. Pectase is the enzyme which hydrolyzes 

 pectins to pectic acid and, in the presence of the calcium salts, 

 brings about the formation of the gel (calcium pectate). Pedinase 

 hydrolyzes pectin (and presumably also pectic acid) to the simple 

 components — the sugars and galacturonic acid — mentioned above. 



Celluloses. — The term cellulose is used in two ways. First, it 

 means a specific chemical compound with distinct properties. 

 Secondly, it is employed in a general sense to include not only the 



