THE LEAF 123 



and remains as reserve material for future use. In some 

 plants the starch is replaced by fatty oils. 



Ferments are substances, probably nitrogenous and of an 

 albuminous nature, which have the power of causing the break- 

 ing up of other substances. The most common of the starch- 

 splitting ferments (all of which are chemically little understood) 

 is diastase, which causes the ti^ansformation of starch into 

 sugar in the seeds, tubers, and stems at the commencement 

 of the vegetative period. The sugar itself often undergoes a 

 change by means of a ferment formed by fungi {eg., by the 

 yeast plant), and termed inveptin. The crystallisable cane-sugar 

 {saccharose), which is obtained from the sugar-cane or the 

 beetroot, is not able to be fermented, but is converted by the 

 action of invertin (also by acids) into grape-sugar {dextrose,, 

 glycose) and into Isevulose, both of which can undergo fermen- 

 tation. A mixture of these two substances (inverted sugar) is 

 of very frequent occurrence in vegetable tissues. The solution 

 of meat, in the case of the so-called carnivorous ^^/a^i^.s, is 

 brought about by "pepsin," which in the presence of acids 

 (formic acid, citric acid) is a very active ferment, and can 

 digest albuminous substances. Whether this vegetable pepsin 

 is identical with the peptic ferment found in the stomachs of 

 animals, which becomes active in the presence of hydrochloric 

 acid, remains to be finally settled. 



In the formation of gums, too, a ferment has been discovered. 

 Gums often occur as transformations of cell-wall substance. 

 The gum arabic {arabin) is of very frequent occurrence, and is 

 looked upon as a combination of a carbo-hydrate with lime. In 

 plum and cherry trees a gum, eerasin, is formed in all the 

 tissues, and is looked upon as a combination of the carbo- 

 hydrate metagummic acid and lime. Basorin, or gum traga- 

 canth, is considered to be a pure carbo-hydrate formed in the 

 genus Astragalus. Together with gums and acids we often 

 find substances consisting also of carbon, hydrogen, and oxygen, 

 which form gelatinous masses on boiling, and are termed 

 pectin substances. They occur largely in the fruits of Ccratonia 

 siliqua (St. John's bread), in tamarinds, and in the pips of 

 apples, quinces, &c. 



Glucosides represent a very largely distributed group of 



