R. BRAITHWAITE ON THE HISTOLOGY OP PLANTS. 239 



cellulose case is homogeneous in the spores of Fungi and Alg^e, 

 leaves of mosses and Hepaticse, &c., or with secondary thickening 

 layers which take an annular, spiral, or netted arrangement, and 

 may also be perforated by closed pores. These cells are coloured 

 blue by chloro-iodide of zinc, or by iodine and sulphuric acid, but 

 occasionally they become lignified, as in the pith and petiole of 

 palms, and medullary rays of oak, fir, &c. ; the iodine then stains 

 them yellow, except the cellulose membrane closing the pores, 

 which is coloured blue, thus showing that it is not lignified, and 

 no doubt on this account retains its original capacity for the 

 .diffusion of fluids throughout the tissues. 



Thickened Parenchym cells are much rarer, but occur in the 

 collenchyma and seed albumen of many plants, and more or less 

 lignified in pith and bark, and in some fruits. 



Collenchyma cells lie under the epidermis, are thin walled and 

 polyhedral, have usually a thickening deposit in the angles, and 

 appear waxy on section, but evidently laminated ; they may be 

 seen in the stems of sorrel, and many umbelliferie. Strongly 

 thickened, unlignified Parenchym cells occur in the albumen of 

 the seeds of lilies and palms, in the coffee berry, ivory nut, date 

 stone, and the seed of Ceratonia siliqua, or Carob bean. 



Lignified Parenchyma is well seen in the pith of Clematis 

 vitalba, and in the pith and bark of Hoya carnosa ; the cells are 

 usually cubical, and often pierced by branched pore-canals. 

 Similar cells are also found in the bark of laburnum, ash, Ficus 

 elastica, &c., and of a more irregular form in the bast of beech 

 and birch, in the shell of the almond, walnut, and Brazil nut, and 

 also in the stony concretions of winter pears ; the thickening often 

 completely obliterating the lumen of the cell. After heating with 

 liquor potassaj the iodine tests colour all the layers yellow. 



II. — Fibre Cells. 



These are almost entirely confined to the vascular bundles, and 

 hence are found in wood and bast tissue, but very rarely in pith or 

 bark. They are distinguished by their great length, and touch 

 each other on all sides, with niore or less oblique faces, so that in 

 outline they are fusiform, pyramidal or conical, and have very 

 rarely any transverse partitions, but we find such in the branched 

 bast cells of Euphorbia. Fibre cells are usually simple, and 

 always more or less thickened, with the secondary and tertiary 



