Vegetable Physiology. 385 



tissues of plants. Cellulose may be recognised by a reaction 

 nearly connected with the above; for when iodine is applied to 

 it either alone or more readily when in conjunction with sulphuric 

 acid, the cell-membrane in certain conditions also assumes a blue 

 colour. Tlie chemical actions which take place here are not yet 

 explained ; but the test is an invaluable one, as it is found to have 

 almost universal application throughout the vegetable kingdom. 

 The cell-membranes of some Fungi appear at present as the most 

 important examples of an exception. The connexion of starch 

 and cellulose is indicated both by their elementary chemical 

 analyses and by the above reaction. The reaction witli iodine is 

 generally most strikingly displayed in the earlier or little altered 

 conditions of the cell-walls. Tlie excessively delicate membrane 

 of the nascent cells of some Confervae (yCEdogonium), and also the 

 very recently-formed layers of thickening in the older cells of 

 these plants, assume a blue colour when treated with iodine alone, 

 much lighter than would be the case with starch, but of the same 

 kind. A similar sensibility to this action of iodine alone exists 

 in the semi-gelatinous layers of thickening in the cotyledons or 

 endosperm of certain seeds (for instance, in certain leguminous 

 plants), where, as was explained above, the thickening matter of 

 the cell-wall is a transitory structure, and is redissolved during 

 germination and appropriated as food, like starch existing in such 

 seeds as cereal grains. But the cell-membranes of parenchymatous 

 tissues generally may be coloured blue by saturating them with 

 strong tincture of iodine, and then wetting tliem with water. In 

 the same conditions of the cell-wall the application of sulphuric 

 acid, in company with aqueous solution of iodine, brings out the 

 blue colour much more readily. This latter reaction affects almost 

 all membranous cellulose, either of fresh structure or in a dead 

 condition, and may be easily observed under the microscope by 

 wetting a icw filaments of cotton wool with somewhat diluted 

 sulphuric acid and adding solution of iodine. 



Many old cell-membranes, together with the harder woody 

 tissues in which the cell-wall has received deposits of solid 

 thickening substance, and the thickened cell-membranes of epi- 

 dermal structures, display a different character when treated with 

 sulphuric acid and iodine. Under these circumstances they 

 assume a deep yellow or brown colour. The observation of their 

 development shows that this character is gradually assumed, and 

 that the same lamellae, which were capable of taking a blue colour 

 when young, lose this peculiarity as they grow older, and even 

 that in some intermediate stages they may acquire a dirty greenish 

 tint. It becomes a point of importance to ascertain the nature 

 of this change, since it might be attributed to a chemical conver- 

 sion of the old cell-wall into a substance different from cellulose. 



