34 CALIFORNIA TANBARK OAK. 



To determine correctly the distribution of tannin in plant tissue, 

 it is best to fix the tannin content in such a manner that it becomes 

 hard, compact, and easily recognizable under the microscope in 

 transverse and longitudinal sections. Tannin hardens into a compact 

 mass when treated with potassium bichromate and, in transmitted 

 light, has an intense red-brown color. 



The ultimate twigs, obtained from growing trees, are cut length- 

 wise through the pith and are allowed to dry for 12 hours in a room 

 of ordinary temperature. Then they are soaked in a solution of 

 potassium bichromate for a week before the sections are cut. The 

 material must be treated before it is sectioned, otherwise the tannin 

 content will be distributed by the knife into elements where it does 

 not naturally occur. The color does not change when the sections 

 are mounted in glycerin, so the sections may be preserved in this way 

 for classroom or other demonstration work. The first 8 or 10 sec- 

 tions should be discarded, because they will include the outer cells 

 coated by tannin that oozed out when the twigs were first cut; and 

 they are likely to be disappointing because they will not show the 

 undisturbed tannin content. The cells in which tannin occurs will 

 be filled with a compact red-brown mass (Pis. IX and X) ; or there 

 may be only a few small red-brown globules, as in the pith ray cells 

 of European alder (Alnus glutinosa, L. Medic.). 



An investigation of this sort shows that tannin is present in the 

 twigs of tanbark oak as well as in the older bark, and that tanning 

 extract could be made from the twigs and smaller branches, as in 

 the case of the eastern chestnut (Castanea dentata). Yet there is 

 but little tannin in the wood-parenchyma elements of the heartwood 

 of tanbark oak, so that tanning-extract can not be got by chipping 

 the wood and subjecting it to tanning-extract processes, as in the case 

 of the chestnut. 



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