STOMATA. 



[ 620 ] 



STOMATA. 



(fig. 204, page 239), and even into the 

 interior of cavities, as on the epidermis of 

 the replum of Crucifera3 (wallflower), and, 

 still more remarkably, on the epidermis of 

 seeds (skin of the walnut). 



In the Liverworts the stomata occur on 

 the fronds and recep- 

 tacles of certain ge- 

 nera (Marchantia, 

 Fegatella, &c., &c.). 

 In Marchantia (fig. 

 451, page 413), they 

 are somewhat circu- 

 lar orifices in the 

 epidermis, guarded 

 by cells arranged in 

 three or four tiers. 

 In the Mosses they 

 are met with on the 

 apophyses or thick- 

 ened summits of the 

 setae bearing the cap- 

 sules, as in Funaria 

 (fig. 266, page 280). 

 The structures here 

 resemble those in the 

 higher plants, as is surface). 



the case also with Magnified 100 diameters. 



those on the leaves of 

 the Ferns. 



In the Flowering Plants the perfect sto- 

 mata appear as roundish or sometimes squa- 

 rish chasms in the epidermal layer, occurring 

 regularly at the meeting angles or sides of 

 four or more epidermal cells, the chasm 

 forming an orifice leading down to a sub- 

 epidermal, intercellular space, and guarded 

 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 convex 

 surfaces adhering firmly to the sides of the 

 epidermal gap. According as the two sto- 

 matal-cells or "pore-cells" or "guard-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 passage into the subepidermal 

 space. Sometimes the " guard-cells " are 

 four in number, in which case they either 

 form two tiers, as upper and lower (Prote- 

 acea?, e. g. Hakea, Protea, &c.), or they are 

 in the same line and parallel, forming inner 

 and outer "guard-cells" (Ficus elastica). 

 In certain coriaceous leaves the stomata are 

 placed on the sides of pits excavated beneath 

 the surface of the leaves, as in Dasylyrion 

 oblongifolium 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 genera] level of the epidermis, and 

 the same is the case with the perfect forms 

 in 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 (PL 

 39. fig. 22), Ficus, Cycas, Hakea, 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, Grevillca, &c. It is important to 

 observe, that in the cases where the " guard- 

 cells " are sunk in the orifice of the epi- 

 dermis, the upper margin of the orifice, 

 formed by the borders of the surrounding 

 epidermal cells, sometimes becomes elevated 

 and even converted into a kind of perforated 

 dome (PI. 39. fig. 22), by development of 

 the cuticular layers (see EPIDERMIS). This 

 might be mistaken for the stomate itself. 

 The same cuticular substance is often deve- 

 loped in mature leaves, not only down 

 over the walls of the stomatd passage, but 

 over the guard-cells, and from thence more 

 or less into contiguous intercellular passages. 

 This may be observed in Euphorbia Caput- 

 MeduscB, Helleborus niger and viridis, Betula 

 alba, Asphodelus luteus, and Cereus, some 

 Aloes, &c. Gasparrini obtained these con- 

 nected processes of cuticular substance, in 

 the form of an isolated coherent piece, by 

 boiling epidermis in nitric acid, which dis- 

 solved the adjoining cell-walls ; these he 

 mistook for peculiar organs, and called them 

 cistomes. Dr. Hooker has described a re- 

 markable 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 (PL 39. fig. 29). 

 This is readily seen in the Equisetaceae, 

 especially E. hyemale, also in the Grasses. 



The mode of development of the stomata 



