II. VON MOIIL ON CELLULOSE. 11? 



layers here, and not only in the thickened layers of the walls of 

 the vessels, but in the delicate membrane which closes the pits. 

 This was the behaviour, for example, in the vessels of Sambucus 

 nigra, Viburnum Lantana, Asclepias syriaca, Buxus sempervirens, 

 Clematis Vitalba, Betula alba, Quercus Robur, and Tilia. The 

 outermost membrane of these vessels behaves in every respect 

 like the outer cell-coat of the prosenchymatous cells of wood, and 

 similar reasons to those which testify that the latter contains 

 cellulose are furnished also by the outer membranes of vessels, so 

 that I may refrain from entering into minute details on this point. 



The researches of Mulder and Harting have already made 

 known that the wall of the Milk-vessels contains cellulose ; and 

 in like manner I only think it necessary to state briefly that the 

 elementary organs of that part of the vascular bundles of Mono- 

 cotyledons which 1 have described under the name of proper 

 vessels (vasa propria) in the Palms, and elsewhere, acquire a 

 beautiful blue colour with iodine after treatment with nitric 

 acid. 



Looking back over the researches here described, it appears 

 clear that the walls of all the elementary organs of vegetables 

 may be brought, by the action of caustic potash or of nitric acid, 

 into a condition in which they assume a blue colour with iodine, 

 and that the only exceptions to this among all the solid structures 

 of plants, are the cuticle, in the strictest sense of the term, and 

 perhaps the intercellular substance of the higher plants. 



The action exerted by potash or by nitric acid upon vegetable 

 membranes is not merely transitory, enduring only while th j 

 action is kept up (as Mulder assumes of the action of sulphuric 

 acid upon cellulose), but is permanent, inasmuch as the mem- 

 branes which have been treated in the above- described way 

 retain the capability of taking a blue colour with iodine after the 

 active substance has been completely removed, as when the 

 nitric acid is neutralized by ammonia. The question now arises, 

 whether the cellulose itself suffers a transformation by the appli- 

 cation of these means, rendering it capable of taking a blue 

 colour with iodine, in the manner starch does, or, whether 

 these means act merely to extract or decompose more or less 

 perfectly the foreign compounds combined with cell-wall, whbh 

 acquire a yellow colour with iodine, and deprive cellulose of the 



