126 ACTION OF IODINE. [BOOK i. 



than that of grains, as lias lately been pointed out. Schleideii 

 had remarked, that when cells are boiled in a ley of caustic 

 alkali till they are dry, they are stained blue by the action of 

 iodine, but lose the property by a prolonged ebullition in 

 water. He did not, however, absolutely admit that by this 

 process, the lining of cells is changed into starch, but he 

 thought it most probably was so, because when weak sulphu- 

 ric acid acts upon vegetable tissue, and iodine is added, a small 

 quantity of iodine of starch is obtained. He supposes that 

 by this operation the primitive membrane of cells is also 

 changed into starch. He also considered that when woody 

 tissue is converted into gum and sugar by the action of sul- 

 phuric acid, that action is secondary, a conversion of it into 

 starch always taking place in the first instance. Schleiden 

 also found that the embryo of Schotia latifolia is completely 

 soluble in water, except the epidermis, and that this solution 

 became blue by the action of iodine ; wherefore he inferred 

 that he had found a plant whose cellular tissue is in its 

 natural state all starch. 



Mohrs object was to pursue this inquiry without calling in 

 the aid of re-agents. When the cells of the cotyledons of 

 Tropseolum niajus are sliced thin, and acted upon by a con- 

 centrated tincture of iodine, they become a fine indigo-blue, 

 but not suddenly ; they are at first yellowish, and it is only 

 after some time they become blue, and then it is through a 

 transition from green, owing to the mixture of yellow and 

 blue particles. In the meanwhile the primitive membranes 

 remain yellow. But if such a slice is placed for a few seconds 

 in strong caustic alkali, and is then washed in water, iodine 

 then colours the cells at once of a clear indigo -blue, and the 

 primitive membrane acquires the same colour, but paler. 

 The cells of Tropseolum are horny at first, and swell up when 

 treated with water; the same kind of texture occurs in 

 Lichens, and Professor Mohl, in consequence, directed his 

 observations to those plants. Although he did not meet with 

 all the success he expected in the course of their examination, 

 nevertheless he found that the shields of many species pre- 

 sented a similar phenomenon; for, in many cases, iodine 

 produced a most beautiful indigo-blue, both in the asci, 



