280 R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 



which do not do so, certain portions of the cortical tissue, by 

 mother-cells, continue to reproduce new tissue of the same kind. 



Cortical tissue consists entirely of parenchym cells, which in 

 leaves usually remain with thin walls, but in the stem are variously 

 modified and may be divided into an inner and outer rind. 



The inner rind is formed of layers of thin- walled spheroidal cells, 

 with their surfaces only slightly in contact, and thus interrupted by 

 apertures of various sizes. Lignification of the cellulose case very rarely 

 occurs, but in a few instances groups of strongly thickened cells 

 are seen, distinguishable by their size and colourless contents ; the 

 ash, beech, laburnum, and hoya afford examples. The contents of 

 the cells of this layer are starch, with the addition of chlorophyl 

 in the more external, and in some instances crystals are also present. 

 Again in milky-juiced plants bast vessels occur, which are con- 

 nected with similar vessels of the bast bundle, and single and 

 grouped bast cells are seen in the inner cortical layer of the leaf 

 stalk of cycads and the bark of many palms. Within this layer 

 also occur the resin, oil, and gum canals peculiar to many plants. 



The outer rind, Collenchyma. — The outer layer consists of rounded 

 parenchyma cells with little or no thickening, but often more or 

 less elongated, or the cells have all irregular strong thickening of 

 their walls or angles, and then constitute collenchyma. When the 

 outer layer of this sub- epidermal tissue consists of thin-walled 

 parenchyma and collenchym cells, the latter are in groups overlying 

 the bast part of the vascular bundle, while the thin-walled cells 

 reach the epidermis, and are opposite the medullary rays ; in these 

 cases the collenchyma is often greatly elongated. The collenchyma 

 has no intercellular spaces, and may take the form of longitudinal 

 strings of cells lying under the epidermis, as in the stem oi Eqnisetum 

 and leaves of Finns; or it may be seen as a connected layer, only 

 perforated by the stomata, in the stems and petioles of many plants, 

 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 lignified, as in Angelica sylvestris, and in others shows 

 porose, netted and spiral thickening, e.g., Sambucus, HeUehorus. 

 The contents are clear or red sap, and also starch and chlorophyl. 



4. — Cork Tissue. — Cork is of much more frequent occurrence 

 in plants than may be generally supposed, and moreover it 

 is Dame Nature's plaister with which she heals up the wounds 

 left by fallen leaves, or if any soft organ be injured, a firm skin of 



