R. ERAITHWAITE ON THE HISTOLOGY OF PLANTS. 



283 



growth in thickness of the shoot, the lenticels lecome expanded 

 into transverse stria?, then cork or bark forms and splits the rind 

 beneath them, the bark scales off, and so they disappear. 



5. — Cuticle. — This covers all the external parts of plants, 

 originates immediately from the primary parenchyma, and consists 

 only of a single layer of cells, and as these continue for a long 

 period capable of reproduction, the cuticle is thus enabled to keep 

 }3ace with the increase in thickness of the stem ; by the full de- 

 velopment of cork it is, however, eventually cast off. It has 

 received the names of epithelium, when thin and first formed ; 

 epihlema, when slightly thickened and covering the subterraneous 

 parts, and both these forms are deficient of stomata; epidermis 

 when fully developed, as we see it on leaves, and the green rind 

 of stems. The cuticle is always composed of flattened or tabular 

 parenchym cells, w^hich in that of seeds acquire a more or less 

 prismatic form, and in quick growing plants become considerably 

 elongated. In most cases the cells of the cuticle are thickened by 

 secondary deposit, which is usually confined to the outer side, and 

 their contents, so long as they are capable of division, consist of 

 protoplasm with the nucleus and cell sap ; in a few cases starch 

 and crystals are also met with. 



Stomata, — These are openings in the cuticle which afford direct 

 communication between the atmospheric air and that contained in 

 the interior of the plant, hence in the higher plants they are found 

 on leaves and green stems growing in air, but not on those sub- 

 merged in water, and their number is very variable in different 

 families, and on different sides of the same leaf ; thus in the 

 mistletoe a square inch of the cuticle is computed to contain 200, 

 but the same extent of surface on the underside of a leaf of 

 Hydrangea qvercifolia contains 160,000, while the upper surface 

 has none. The stomata are quadrate or oval in outline, and originate 

 in an epidermis cell which divides into two sister cells of a semi- 

 lunar form, and these become the guard cells which protect the 

 orifice. In some of the lower plants, as Marchantia, the guard cells 

 are absent, and the stoma is an open channel built up of several 

 series of cells. 



The stomata are situated on both sides of a leaf, but usually their 

 most frequent seat is on the underside of aerial and upper side of 

 floating leaves, and their arrangement may be in rows, as iii Fi?ius, 



