STOMATA. 



L 742 ] 



STOMATA. 



Epidermis of the White 



ded a little below the upper edge, more 

 deeply, or even at the bottom, by usually 

 two semilunar cells, applied together by 

 their flat faces, but not coherent, their con- 

 vex surfaces adhering firmly to the sides 

 of the epidermal gap. According as the 

 two stomatal cells 

 pore-cells or guard Fig. 706. 



cells are distended 

 or collapsed, their 

 flat faces approach 

 or retreat from each 

 other, in the latter 

 case leaving a slit- 

 like orifice leading 

 from the outer pas- 

 sage into the subepi- 

 dernial space. Some- 

 times the guard 

 cells are four in 

 number, in which 

 case they either form 

 two tiers, as upper 

 and lower (Prote- 

 aceae, e. g. Hakea, 



Protect, &C.), Or they Lil / with stomata st (lower 



are in the same line B ace ?' , in 

 and parallel, form- Magnitied 10 diamefcer8 ' 

 ing inner and outer guard cells (Ficus 

 elasticci). In certain coriaceous leaves the 

 stomata are placed on the sides of pits ex- 

 cavated beneath the surface of the leaves, 

 as in Dasylirion longifolium and Nerium 

 Oleander. 



A considerable difference exists between 

 the appearances presented by vertical sec- 

 tions of the epidermis of leaves made so as 

 to pass through the stomata. In young 

 leaves the guard cells are little (if at all) 

 below the general level of the epidermis ; and 

 the same is the case with the perfect forms 

 of various herbaceous plants in which the 

 leaves are of membranous texture. In other 

 cases, as in the Hyacinth, Iris, Narcissus, 

 Equisetum, &c., the guard cells are found at 

 a very early period quite beneath the layer 

 of epidermal cells, attached as it were 

 under the passage communicating with the 

 air. The same occurs very frequently in 

 the stomata of coriaceous leaves, as in Aloe 

 (PI. 48. fig. 22), Ficus, Cijcas, Hakca, Protea, 

 &c. In other instances also in leathery 

 leaves, the guard cells appear more or less 

 elevated above the general level of the 

 epidermal cells, as in some species of Leu- 

 cadendron, Grevillea, &c. It is important to 

 observe that in the cases where the guard 

 cells are sunk in the orifice of the epidermis, 



the upper margin of the orifice, formed by the 

 borders of the surrounding epidermal cells, 

 sometimes becomes elevated and even con- 

 verted into a kind of perforated dome (PI. 

 48. fig. 22) by development of the cuticular 

 layers (see EPIDERMIS). This might be 

 mistaken for the stoma itself. The same 

 cuticular substance is often developed in 

 mature leaves, not only down over the walls 

 of the stomatal passage, but over the guard 

 cells, and thence more or less into contiguous 

 intercellular passages. This may be observed 

 in Euphorbia Caput-Mcdusce, Helleborus 

 niyer and viridis, Betula alba, Asphodelus 

 luteus, and Cereus, some Aloece, &c. Gas- 

 parini obtained these connected processes of 

 cuticular substance, in the form of an 

 isolated coherent piece, by boiling epidermis 

 in nitric acid, which dissolved the adjoining 

 cell-walls : these he mistook for peculiar 

 organs, and called them, cystomes. Hooker 

 has described a remarkable form of stomata 

 in the parasitical plant Myzodendron. 



In those plants in which the epidermis 

 becomes infiltrated with siliceous matter, the 

 walls of the stomatal pore and the guard 

 cells become imbued with it, and a sili- 

 ceous skeleton of the structure remains after 

 the organic matter has been removed by 

 nitric acid and burning (PI. 48. fig. 29). 

 This is readily seen in the Equisetacese, 

 especially E. hyemale, also in the Grasses. 



The mode of development of the stomata 

 appears to be uncertain. Mohl and other 

 authors assert that the guard cells origi- 

 nate from one of the cells of the subepi- 

 derinal tissue, which is pushed up into a 

 vacancy formed by the separation of the 

 epidermal cells at certain points. This cell 

 is next divided into two, which become 

 free from each other in the line of the new 

 partition then formed. Nageli and others 

 assert that the guard cells are originally 

 constituent cells of the epidermal layer, 

 which become subsequently displaced down- 

 wards (or upwards), and undergo special 

 development analogous to that just de- 

 scribed. In the Iridaceae,Equisetace8e, and 

 some other plants, the process is more com- 

 plicated. 



The stomata are generally largest upon 

 succulent leaves, smallest on hard and 

 leathery kinds ; their form and number are 

 most varied, both in different plants and on 

 different parts of the same plant. They 

 abound most on the lower face of leaves ; 

 but it has been mentioned that they are not 

 found on submerged organs, and on floating 



