GALL^ HALEPENSES. 597 



central cavity, sometimes nearly \ of an inch in diameter, which served 

 as a dwelling for the insect, is lined with a thin hard shell. If the 

 insect has perished while still very young, the central cavity and the 

 aperture contain a mass of loose starchy cellular tissue, or its pulverulent 

 remains : if the insect has not been developed at all, the centre of the 

 gall is entirely composed of this tissue. 



Microscopic Structure— The cellular tissue of the gall is formed 

 in the middle layer of large spherical cells with rather thick porous 

 walls, becoming considerably smaller towards the circumference. The 

 outermost rows are built up of cells having but a very small lumen and 

 comparatively thick walls, so that they foi-m a sort of rind. Here and 

 there throughout the entire tissue, there occur isolated bundles of vessels 

 which pass through the stalk into the gall. Towards the kernel, the 

 parenchyme gradually passes into radiaUy-extended, wider, thin-walled 

 cells, the walls of which are marked with spiral stria?. The hard shell 

 of the chamber^ is composed of larger, radially-extended, thick- walled 

 cells, with beautifully stratified porous walls. On the inner side of this 

 shell there are found, after the escape of the insect, the remains of the 

 starchy tissue already mentioned, which originally filled the chamber 

 and had been consumed by the insect as nourishment. 



The parenchyme-cells outside the shell contain chlorophyll and 

 tannin; the latter is in transparent, colourless, sharp-edged masses, 

 insoluble in benzol, but dissolving slowly in water, quickly in alcohol. 

 Thin slices soaked in glycerin appear after some time covered with 

 beautiful crystals of gallit acid. The thick -walled cells (stone-cells) 

 and the neighbouring striated cells, are rich in octahedra of calcium 

 oxalate. The tissue of the gall situated within the shell of thick-walled 

 cells contains starch in large, compressed, mostly spherical granules ; 

 also isolated masses of brown resin. Besides these, there appeal's to be 

 in this part of the tissue an albuminoid compound. 



Chemical Composition — The rough taste of galls is due to 

 their chief constituent, Tannic or Gallo-tannic Acid, C"ff"0', or 

 r*TT"^0TTyP001T ) 

 P^H'/OTT 3pn 1 ^' ^^ ^yP® ^^ ^ numerous family of substances to 



which vegetables owe their astringent properties. Tannic matter was 

 long supposed to be of one kind, namely that found in the oak gall, 

 but the researches of later yeare have proved the tannin of difierent 

 plants to possess distinctive charactei-s : hence the term gallo-tannic 

 acid to distinguish that of galls, from which it is principally derived. 

 It was however shown by Stenhouse as far back as the year 1843, 

 again in 1861, as well as by still more recent unpublished experiments, 

 that the tannic acid found in Sicilian sumach, the leaves of Rhus 

 Coriaria L., is identical with that of oak galls. Lowe in 1873 came to 

 the same conclusion. The best oak galls yield of this acid, from 60 to 

 70 per cent. 



Gallic Acid is also contained in galls ready-formed to the extent of 

 about 3 per cent. Free sugar, resin, protein-substances, have also been 

 found. Neither gum nor dextrin is present. 



Commerce — The introduction into dyeing of new chemical sub- 



^ Couche protectrice oi Lacaze-Duthiere — — Ann. des Science>< Xat., Bot. xix. (1853) 

 Jtecherches pour serrir a Vhidoire des gallef. 273-354. 



