PROGKESS OF MICROSCOPICAL SCIENCE. 157 



and are opposite tlie medullary rays ; in these cases tlie collencliyma is 

 often greatly elongated. The coUenchyma has no intercellular spaces, 

 and may take the form of longitudinal strings of cells lying under the 

 e^jidermis, as in the stem of Equisetum and leaves of Piniis ; or it may 

 be seen as a connected layer, only perforated by the stomata, in the 

 stsms and i')etioles of many j^lants, and also in many leaves as a well- 

 developed layer, e. g. in the vine, elder, and begonia. The cellulose 

 case is usually soft, but in a few instances ligniiied, as in Angelica 

 sylvestris, and in others shows porose, netted and spiral thickening, e. g. 

 Sambuciis, Helleborus. The contents are clear or red sap, and also 

 starch and chlorophyll. 



Of the second, or Cork Tissue, he says, that it is of much more 

 frequent occurrence in jilants than may be generally supposed, and 

 moreover it is Dame Nature's jjlaster with which she heals uj) the 

 woimds left by fallen leaves, or if any soft organ be injured, a fii-m 

 skin of new cork cells rapidly protects the sound tissues from the 

 outer damaged structures. The walls of this tissue are highly resistent 

 to the various reagents, behaving in this respect like the cuticle, being 

 also elastic and with difficulty permeated by air or water ; the cells 

 are rectangular without intei'cellular spaces, are arranged in rows at 

 right angles to the surface, and mostly lose their contents and become 

 filled with air ; the cell membrane is but moderately thickened, and is 

 soon altered into cork. Primary cork tissue arises later than the other 

 elements, and the altered parenchym cells, which become the mother 

 cells of cork, may be either cells of the cuticle, of the collenchyma, of 

 the inner rind, or of tlie parenchyma of the bast part of the vascular 

 bundle ; these mother cells repeatedly divide, and of these newly- 

 arising cells in each radial series, the inner one remains thin-walled, 

 filled with protoplasm, and constantly forming new cells by division, 

 and this is termed the corJc — camhium or phellogen layer, while the 

 outer becomes suberified and permanent. Generally the cork first 

 commences at single points, but these gradually coalesce, and the phel- 

 logen forms a continuous layer, from which constantly new cork layers 

 are being jjushed outward and constiti;te the periderm. Sometimes the 

 cork cells become altered in form, and the periderm consists of alter- 

 nate lamiufe of different shaped cells ; this is seen in the cork of the 

 cork-oak, and of birch. As examples of cuticular development of cork 

 we may mention the apple tree, oleander, mountain-ash and Viburnum 

 Lantana ; here the epidermal cells divide into two daughter cells, the 

 uj)per of which with the cuticular layers and tertiary cellulose case 

 become suberified, and the lower becomes the mother cell of the next 

 cork formation. In the greater jDart of our trees, as in the maple, 

 beech, oak, elm, plum, horse-chestnut, elder, &c., the collenchym cells 

 lying next under the cuticle become the mother cells of cork ; and 

 among the number of plants in which the cork tissue arises deeper 

 below the cuticle, but yet within the outer rind, Ficus elastica and 

 Mohinia pseudacacia are well suited for observation ; here the cells 

 of the second or third row of collenchyma become the mother 

 cells of the cork. In the bramble and currant bushes the cork tissue 

 arises in the inner rind, and indeed it is the cells next to the vascular 



