122 



ANATOMY. 



formed of two curved cells whose concavities face each other. These two small lip- 

 like cells are termed stomata. Stomata, though epidermal org?/ns, differ from the 

 epidermis in that their cells are much smaller, and nearly always situated below 

 those of the epidermis ; they further present different contents, and especially granules 

 of chlorophyll ; whence they may be regarded as intermediate between the epidermis 

 and the subjacent parenchyma. 



Stomata are variously distributed over the surface of the leaves : usually solitary, 



often arranged in series, some- 

 times crowded in the base of 

 a cavity (as in some Proteacece, 

 figs. 678, 679). Their number 

 varies : the Iris contains 

 12,000 in a square inch; the 

 Pink, 40,000; the Lilac, 

 120,000. When moistened, 



their lipS SWell and become 679. Part of a Banksia leaf, 



presenting three sections 



more curved, and hence gape ; parallel to the lower snr- 



3 r ' face, and at different 



when dry, they shorten and de P ths (mag.). 

 close. 



Stomata always correspond to intercellular passages, and are found on the 

 ordinary leaves of Phamogams, principally on their lower surface, on stipules, on 



P_ ^ herbaceous bark, calyces, and ovaries ; 



they are wanting on roots, rhizomes, non- 

 foliaceous petioles, most petals, and seeds ; 

 ncotyledons, and submerged aquatic plants, 

 which have no epidermis, equally want 

 stomata. 



If a fragment of a stem or leaf be 

 macerated, the cellular tissue beneath 

 the epidermis is rapidly destroyed, and the latter divides into two layers, an 

 external epidermis proper, and a very thin membrane (fig. 680), moulded on the 

 epidermis and extending even over its hairs, which are sheathed in it like fingers in 

 a, glove (P) ; it presents openings (P) corresponding to the stomata. Brongniart has 

 called this membrane the cuticle (little skin] ; it is not cellular, like the epidermis 

 which it covers. 



678. Vertical section of part of a Banksia 

 leaf (mug.)' 1 



80. Cabbage. Cuticle. 



1 Fig. 678 is a section perpendicular to the thickness 

 of the leaf, showing : 1, on the upper and lower faces 

 two layers of epidermal cells ; 2, fibro-vascular bundles 

 to the right and left, cut perpendicularly to their length ; 

 3, on the lower face, a depression, clothed with hairs, 

 and pierced by stomata which communicate with the 

 interstices of a very loose cellular tissue. Above this 

 tissue, the upper half is a mass of elongated and erect 

 cells, perpendicular to the epidermis. 



* Fig. 679. Three sections parallel to the plane of the 



leaf, carried through three such depressions, each circum 

 scribed by the fibro-vascular bundles of the nerves. In 

 the cavity at the bottom of the figure, the section has 

 carried away the hairs clothing the walls of the depres- 

 sion, leaving the stomata and epidermal cells visible ; 

 in the cavity on the right the loose cellular tissue which 

 underlies the stomata of the epidermis is seen through 

 the latter ; in the third depression the section has re- 

 moved all but this subjacent tissue with its intercellular 



