528 SUMMARY OF CURRENT RESEARCHES RELATING TO 



These formations of duramen are commonly j)roduced in tlie first 

 place by derivatives from the solid contents, especially starch, filling 

 up all the elements of the wood. These products are formed in the 

 affected part, as well as being conveyed from the adjoining normal 

 parts. These products cause an increase in the mass of the duramen, 

 so that its weight exceeds that of the alburnum. These substances 

 must be in a fluid or half-solid state when deposited in the tracheal 

 elements, since they reproduce perfectly the inner structure of the 

 walls of the cells and vessels. As long as these substances are con- 

 tained in the parenchymatous elements they are rich in tannin, so that 

 they seem to constitute a link between the contents of the duramen 

 and starch. In addition there also occur products of decomj)osition 

 of a different kind, which, at least after they have been deposited for 

 some time, considerably increase the power of resistance of the 

 duramen. Nitric acid, or Schulze's solution, and potash or soda-lye, 

 used in succession, remove nearly comijletely the contents of the 

 duramen, excejit in the case of Diospyros ; and the uninjured cell-wails 

 are then exposed with their thickening-layers, and distinctly exhibit 

 the cellulose-reaction. If the deposition has lasted for a very long 

 period, the cell-walls are themselves partially destroyed, and the pro- 

 ducts of their decomposition mingle with the cell-contents. The 

 contents of the duramen vary in their composition according to the 

 species ; in Prumis, for example, and the Amygdalese generally, they 

 consist chiefly of gum ; in the Ooniferje of resin ; in Si/ringa of 

 resinous substances. Their mode of origin is the same in all cases. 

 The function of the duramen, especially in stumps, is the protection 

 of the subjacent tissue from injurious external influences. The same 

 effect is produced by the thyllae which occur in many plants, and by 

 deposits of calcium carbonate. 



Polarization-phenomena of Vegetable and Artificial Colloid- 

 cells.* — N. J. C. Miiller gives the following as the results of a series 

 of observations on the polarization of vegetable colloids and of 

 gelatine : — 



Hollow spheres in vegetable colloids and in gelatine have a nega- 

 tive tension in the marginal layer ; that is, they behave as if their 

 marginal layer were made rigid by expansion of the cavity. Solid 

 spheres, on the other hand, such as the natural spheroids of starch 

 and inulin, have a positive tension — that is, they exhibit a rigidity 

 due to compression. All cell-walls in the interior of the plant behave 

 as if they were made rigid under negative tension. Polyhedral and 

 sjjherical cells are optically uniaxial ; cylinders and prisms optically 

 biaxial. Transverse sections of all correspond to the optical section 

 of the marginal layer of a hollow sphere. The cuticular layer and 

 cork-membrane behave in the reverse way ; they correspond to a 

 colloid mass made rigid under compression, so that a calotte cut out 

 of an extine-intine mass of the epidermis behaves precisely like a 

 circular section of a glass tube. All vegetable colloids adapt them- 

 selves to cylindrical cells, which correspond, in their optical properties, 



* Ber. Deutscb. Bot. Gesell., i. (1883) pp. 77-83. 



