R. BRAITHWAITE ON THE HISTBOLOGY OF PLANTS. 281 



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 intercellular spaces, are arranged 

 in rows at right angles to the surface, and mostly lose their con- 

 tents and become filled with air; the cell membrane is but moder- 

 ately 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 the 

 parenchyma of the bast part of the vascular bundle ; these mother 

 cells repeatedly divide, and of thesenewly 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 cork — 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 phellogen forms 

 a continuous layer, from which constantly new cork layers are 

 being pushed outward and constitute i]it periderm. Sometimes the 

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

 alternate laminae of different shaped cells ; this is seen in the cork 

 of the cork-oak, and of birch. As examples of cuticular develop- 

 ment of cork we may mention the apple tree, oleander, mountain- 

 ash and Viburnum Lantana, here the epidermal cells divide into two 

 daughter cells, the upper 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 part 

 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 Rohinia 2)seudacacia 

 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 bundle 

 which become the mother cells, so that all the young cortical 

 tissue becomes pushed off by the cork tissue. 



In many cases it is not solely cork cells proceeding from the 



