This arrangement is a method of checking transpiration by creating a still atmosphere above the pore of the stoma, so that water vapour collects in it and diminishes the further outflow of vapour.
C/i, epidermis; st stoma; me,, mesophyil; pal, palisade; spa, spongy tissue; Isp, inteicellular space; wi., water tissue; x, xylem; p/i, phioem; Phil, phloeoterma; sri, scierenchyma.
The amount of watery vapour in the air passing through a stoma has no effect upon it, as the surfaces of the guard cells abutting on the air chamber are strongly cuticularized, and therefore impermeable.
If the membrane is of some impermeable substance, like gold leaf, the hyphae cannot dissolve its way through, but the tip finds the most minute pore and traverses the barrier by means of it, as it does a stoma on a leaf, We may hence conclude that a parasitic hyphae pierces some plants or their stomata and refuses to enter others, because in the former case there are chemotropically attractive substances present which are absent from the latter, or are there replaced by repellent poisonous or protective substances such as enzymes or antitoxins.
Rocco, "Sull' origine del Mito di Caronte," in Rivista di stoma antica, ii.
Above each cavity is situated a so-called water-sloma, no doubt derived phylogenetically from an ordinary stoma, and enclosed by guard-cells which have nearly or entirely lost the power of movement.