386 Vegetable Physiology. 



or it might be supposed to arise from the cellulose layers becoming- 

 infiltrated by substances which disguise its reactions. The latter 

 view is that which is most favoured by the evidence yet obtained. 

 It is found that the structures most obstinately refusing to assume 

 a blue colour with sulphuric acid and iodine, may be brought 

 into a condition, without losing their recognisal^le anatomical 

 characters, in which they react with tincture of iodine (without 

 sulphuric acid), like young cellulose, A difference presents 

 itself here in the behaviour of two classes of tissues, very dif- 

 ferently placed as regards their relation to atmospheric agencies, 

 which may perhaps influence their characters. The structures 

 behmging to the epidermis present a marked difference from those 

 belonging to the internal woody tissues. 



In tlie cells of well-developed epidermis, cork and the cork-like 

 tissues of bark, iodine and sulphuric acid colour the cell-mem- 

 brane brownish-yellow only. But if sections of these structures 

 are soaked for a long time, or boiled for a short time, in strong 

 solution of potash, and well washed, they are brought into a state 

 (the structure being undestroyed) resembling young cellulose, for 

 if soaked in tincture of iodine, dried, and then wetted with water, 

 they acquire a bright blue colour. Potash will not bring about 

 this change in the cells of wood and fibrous tissues, but nitric 

 acid will ; nitric acid on the other hand will not, as a rule, affect 

 the epidermal tissues. Sections of wood, liber, or other struc- 

 tures, where the cells possess firm ligneous walls, assume a yellow 

 or brown colour with sulphuric acid and iodine. When such 

 sections are boiled in nitric acid so long that the yellowish tint, 

 which this acid gives at fust, gives place to a bleaching, the tissue 

 is changed, as in the above process with potash. After neu- 

 tralizing the acid with ammonia and washing away the salt then 

 formed, — saturating the object with tincture of iodine, drying, and 

 then adding water, brings out the characteristic blue colour of 

 cellulose. This possibility of bringing all the solid tissues of 

 plants, in any condition of development, into a state in which they 

 exhibit the reaction of pure cellulose, still retaining their ana- 

 tomical structure, affords good ground for the assumption that cellu- 

 lose forms the universal basis of the substance of vegetable tissues, 

 and that the diverse conditions met with in the different parts 

 of plants, and at different ages, depend upon different degrees of 

 consolidation of the substance, admitting a more or less free access 

 to the iodine, and, in the older tissues, to the infiltration of the 

 membranes with foreign substances, which still more strongly 

 oppose the interference of the iodine, and themselves give the 

 yellow colour which iodine and sulphuric acid produce. 



It may be worth while to advert briefly here to a certain real 

 metamorphosis of the cellulose membrane. The superficial 



