58 HOW CROPS GROW. 



designation of gums. The best known are Gum Arabic, 

 the gums of the Peach, Cherry and Plum, Gum Traga- 

 canth and Bassora Gum, Agar-Agar and the Mucilages 

 of various roots, viz., of mallow and comfrey ; and of 

 certain seeds, as those of flax and quince. 



Gum Arabic exudes from the stems of various species 

 of acacia that grow in the tropical countries of the East, 

 especially in Arabia and Egypt. It occurs in tear-like, 

 transparent, and, in its purest form, colorless masses. 

 These dissolve easily in their own weight of water, form- 

 ing a viscid liquid, or mucilage, which is employed for 

 causing adhesion between surfaces of paper, and for 

 thickening colors in calico-printing. 



Gum Arabic is, however, commonly a mixture of at 

 least two very similar gums, which are distinguished by 

 their opposite effect on polarized light and by the differ- 

 ent products which they yield when boiled with dilute 

 acids. 



Cherry Gum. The gum which frequently forms 

 glassy masses on the bark of cherry, plum, apricot, peach 

 and almond trees, is a mixture in variable proportions of 

 two gums, one of which is apparently the same as occurs 

 in gum arabic, and is fully dissolved in cold water, while 

 the other remains undissolved, but 

 Bwollen to a pasty mass or jelly. 



Gum Tragacanth, which comes 

 to us from Persia and Siberia, has 

 much similarity in its properties 

 to the insoluble part of cherry 

 gum, as it dissolves but slightly in d ' ""JC/ JC^ 

 water and swells up to a paste or 

 jelly. 



The so-called Vegetable mucilages 

 much resemble the insoluble part 

 of cherry gum and are found in 

 the seeds of flax, quince, lemon, and in various parts of 

 many plants. 



