METABOLIC PROCESSES IN THE PLANT. 333 



from twigs of Tilia parvifolia about 5 mm. thick. The air-con- 

 taining cells of the pith are large. They are grouped in the form 

 of rosettes round single small cells which contain tannin, starch, 

 or cluster crystals. The gum reservoirs, appearing as cavities, 

 lie in the outer parts of the pith. The periphery of the pith, into 

 which the primary masses of wood project, is formed of small- 

 celled parenchyma whose elements contain tannin or starch. 



If we examine under the microscope a transverse section from 

 a tuber of Orchis mascula, or O. Morio, the parenchyma in which 

 the vascular bundles are scattered is seen to consist of small cells 

 which contain starch, and large cells which are very rich in 

 mucilage. If we extract pulverized Orchid tubers with cold 

 water, or treat commercial salep with cold water, we shall obtain 

 after filtration a clear fluid. Addition of alcohol to this causes 

 precipitation of white flocculent masses of Orchid mucilage, in- 

 soluble in alcohol. If we evaporate the mucilaginous solution pre- 

 pared as described, and treat the residue with iodised Potassium 

 iodide solution and Sulphuric acid, it takes on a violet to blue 

 coloration. The mucilaginous masses in Orchid tubers are thus 

 not gum but true vegetable mucilages. 1 



1 The literature on gums and mucilages is given in Sachsse, Die Cliemie und 

 Physiologic der Farbstoffe, Kohlehydrate, etc., Leipsic, 1877, p. 161. 



133. Tannic Acids. 



Tannic acids appear to function in plants chiefly as a means of 

 protection against the attacks of animals, and as antiseptics. It 

 is quite in accordance with this that in many cases it is precisely 

 the peripheral tissues which are specially rich in Tannic acid. 

 The best test for Tannic acid is Potassium bichromate, 1 and we 

 will make use of it to determine the distribution of Tannic acid in 

 a shoot of Corylus Avellana. We first prepare transverse sections 

 from a twig about 4 mm. thick. The periderm is followed by 

 collenchyma, this by cortical parenchyma; then comes a ring of 

 strongly thickened sclerenchyma cells, followed by the bast with 

 its scattered bast fibres, and lastly the wood. If we put longi- 

 tudinally halved .pieces of Corylus twigs (I investigated twigs 

 4 mm. thick, cut in November) for a few days into a 10 per cent, 

 solution of Potassium bichromate, and then examine under the 

 microscope delicate transverse sections, the presence of Tannic 



