16 POEES ARE PITS. [BOOK i. 



unthickened membrane of young cells, when coloured blue 

 by iodine and sulphuric acid, is seen to be perforated like 

 a sieve by a great number of small pores, through which the 

 light appears bright and uncoloured ; the cells of the pith of 

 Asclepias syriaca, Hoya carnosa, Ricinus communis, of the 

 bark of Euphorbia Caput medusae, of the wood of Asclepias 

 syriaca, and Clematis Vitalba, are especially named in relation 

 to this. Harting states that in the old medulla-cells with 

 thickened walls of a great many dicotyledonous trees, e. g. 

 ^Esculus Hippocastanum, Syringa vulgaris, Rosa canina, 

 Sophora japonica, there are, among the canals of the dots 

 closed by a membrane, others which are quite open ; and from 

 his investigations he was led to the conclusion that these open 

 pores are not the result of the absorption of the membrane 

 closing the canal, but that they are really remains of pores 

 occurring in the young cell, which have not, like the others, 

 become closed at a subsequent period. (Annals of Natural 

 History, xviii. 146.) Mohl's reply to this seems conclusive 

 " I had already/' says this admirable phytotomist, " in cells 

 coloured blue by iodine, often seen very bright dots, which 

 appeared like real orifices, but I always believed that I saw a 

 closing membrane ; yet as I might have been deceived in 

 my earlier observations, I submitted this to a new investi- 

 gation. In the first place, however, I must remark that I do 

 not wholly approve of the mode of examination with iodine 

 and sulphuric acid, chosen by Harting and Mulder ; a deep 

 blue colouring of the young cell- membrane is indeed obtained 

 by this means, but in fact this deep colour is not advan- 

 tageous, as will hereafter appear ; moreover, when too strong 

 an acid is employed, a considerable expansion of the cell- 

 membrane is readily caused, by which the dots may be 

 closed ; this indeed cannot give rise to a delusion in reference 

 to the presence or absence of a closing membrane, but 

 renders the making of a new preparation necessary. Both 

 evils are avoided when no sulphuric acid is used ; but the 

 cell- wall is coloured blue by the application of very concen- 

 trated tincture of iodine, and subsequent moistening with 

 water. In this way we are not exposed to the risk of pro- 

 ducing a mechanical alteration of the cell-membrane, and 



