38 ELEMENTS OF PLANT ANATOMY. 
the balsam being a mixture of resins and ethereal oils, together 
with aromatic acids. Gum resins are a mixture of resins, gums, 
and ethereal oils. They may originate in cells from which they 
are excreted into intercellular spaces, or they may arise from the 
breaking down of the cell wall and are thus often found mixed 
with unchanged parts of plant tissue. Caoutchouc is a sub- 
stance related to resins and originates in certain cells called milk 
tubes where it forms an emulsion with the water of the sap. 
13. Dextrin, Inulin and Gums. 
Dextrin is a substance very closely allied to starch and is 
found dissolved in the sap of many plants. Sinistrin and Inulin 
Fig. 19. 
Inulin crystals. 4 from tuber of Dahlia variabilis. B from Helianthus tuberosus. Crystals 
appear after the material has been treated with strong alcohol. x 500.—(4eed. to Sachs.) 
also occur in solution in sap, the latter in considerable quantities 
in the underground parts of the order Compositae, where it takes 
the place of starch. It may be precipitated as crystals of charac- 
teristic form, by the use of alcohol. There are several kinds of 
eums, some of which arise from the breaking down of the cel- 
lulose of the wall, others as secretions which are developed in 
certain cells designed for this purpose. 
e* 
