FORM OF THE PLANT-CELL. 



71 



The gradual development of these external cells is attended with an 

 irregular nourishment of their lateral walls, whereby round or pointed 

 projections are formed, which are received into the concavities of the 

 surrounding cells, so that a waving line appears. This causes a great 

 variety in the appearance of the cells, according to the number of the 

 individual cells, the size of the wavy bulgings, and the roundness or 

 sharpness of their projections (fig. 75.). These cells are distinguished from 

 those lying under them, by the presence of a transparent colourless or 

 coloured fluid, but never, as has been erroneously stated, by containing 

 air. The configuration of the cell-walls of the epidermis is very varied. 

 A common phenomenon is, that their walls are, above and laterally, 

 thicker than they are at the lower part, where they lie upon the paren- 

 chyma (figs. 68, 69.), as seen in the seeds of the Asparagus offlcinalis. 

 Spiral formations abound in it, both with jelly in the seeds of Hydrocharis 

 Morsus ranee, and without jelly in the peri cap of Salvia verticillata.* The 

 epidermis-cells are frequently porous, sometimes on the side where 

 they touch each other, as in the leaves of Epidendrum elongatum ; or 

 where they touch the parenchyma-cells, as in the stem of Melocactus; 

 they are least frequent externally, but this form presents itself in the 



leaves of Abies. In these leaves 

 every one of the thick-walled ex- 

 ternal cells possesses three or four 

 rows of porous canals, which run 

 externally, and terminate in a small 

 round cavity. The same thing is 

 seen in Cycas (fig. 76.). The cells 

 of the epithelium are placed so 

 close to one another, that no inter- 

 cellular passages are found opening 

 between them. When epithelium is converted into epidermis in the 

 air, it happens that the cells soften at their margins, and thus form inter- 



* Schleiden, Beitrage zur Phylogenesis, Miiller's Archiv, 1838. See Taylor's 

 Scientific Memoirs. 



74 Epidermis (a) of the seed of Lupinus rivularis, composed of long, very much 

 thickened, cells. Under these is a layer of entirely separate cells, and beneath them 

 parenchyma. 



75 Epidermis of the nectary of Goldfussia anisophylla. The cells are exceedingly 

 flat and irregular. Section parallel with surface. 



r6 Perpendicular section of the surface of a leaf of Cycas revoluta. The epidermis- 

 cells (6) are laterally and externally porous. They are covered above by a layer of 

 secreted matter (a). 



