Stoma (botany)

“ And yet there is still one more act to play and the stoma have much to do with it. For this process of photosynthesis and the closely related one of supplying food and water to the leaf cannot go on without respiration, which is quite another thing. In plants respiration or breathing has no more to do with digestion than it does in man. Digestion in man is not unlike photosynthesis in plants, except that plants make food in the process while men destroy it. But plants must breathe just as we do, and, as we need oxygen to renew our vital processes, so do they. ”

“ The opening and closing of the stoma by the guard cells is a nicely balanced operation dependent upon root pressure, turgor, and atmospheric conditions. Guard cells have, because of this, been much studied in spite of the fact of their microscopic size. We now know that they allow greater openings during the night and reduce them during the day. ”

“ The [319] stomata are often situated in furrows on the underside of the leaf, and hence cause a striped appearance. They are all American (525 species) , especially from S. America, where they live partly as epiphytes on trees, partly in the clefts of rocks, often on the steepest slopes, to which they firmly attach themselves by aerial roots; some are terrestrial. The stem is seldom tree-like or many metres in height (Puya, in Chili; Hechtia, in Mexico) . The inflorescence is a terminal spike, raceme, or panicle, often with large and brightly-coloured floral-leaves. ”

“ The stomata are more numerous on the lower surface, and are surrounded by ordinary epidermal calls.
The mesophyll is composed of palisade tissue on the adaxial side and of arm-palisade tissue on the abaxial side. Internal secretory organs do not occur in the leaf. They are represented in the axis by secretory cells with tanniniferous contents in the soft bast and pith. Oxalate of lime occurs in the form of numerous clustered crystals near the veins in the leaf. The clustered crystals occur in a layer of cells outside the pericycle and in the pith of the axis. ”

“ They are tough, to withstand battering by waves; the stomata are situated, not on the lower side of the leaf, as in land plants, but on the upper side, where they are in contact with the atmosphere; and the upper surface is waxy or oily, so that it is not wetted and the stomata are not blocked. Changes of water-level are met by means of long flexible stems, rising not vertically from the root, but at an angle, so that the leaves can rise with a rise of water-level. But not all the plants are anchored to the bottom. ”

“ It has been estimated that 150,000 stomata or more are often found per square inch on the under side of the leaves of ordinary cultivated plants. The stomata or breathing-pores are so constructed that they may open and close very readily. In wilted leaves they are practically closed; often they also close immediately after a rain; but in strong sunlight they are usually wide open. It is through the stomata that the gases of the air enter the plant through which the discarded oxygen returns to the atmosphere. ”

“ In young plants these guard-cells are very little below the surface of the leaf or skin, but in others they are sunk quite beneath the layer of cells forming the outer coat of the tissue. There are other cases where they are slightly elevated above the surface.
Stomata are found chiefly in the green portions of plants, and are most plentiful on the under side of leaves. It is, however, worthy of notice, that when an aquatic leaf floats on the water, the mouths are only to be found on the upper surface. ”

“ When the roots fail to supply to the plant sufficient water to equalize that transpired by the leaves, the plant wilts. Transpiration from the leaves and delicate shoots is increased by all the conditions which increase evaporation, such as higher temperature, dry air, or wind. The stomata open and close, tending to regulate transpiration as the varying conditions of the atmosphere affect the moisture content of the plant. However, in periods of drought or of very hot weather, and especially during a hot wind, the closing of these stomates cannot sufficiently prevent evaporation. ”

“ Sunken stomata often have valve-like projections of cuticle which reduce the opening also. Finally, in a few plants, water loss in times of drouth is almost completely prevented by closing the opening with a wax excretion.
5. Modifications in the chlorenchym. A decrease in the size and number of the air passages in the leaf renders the movement of water-laden air to the stomata more difficult, and effects a corresponding decrease in transpiration. The increase of palisade tissue, though primarily dependent upon light, reduces the air-spaces, and consequently the amount of water lost. ”

“ Their guardian cells or lips, which are soft and delicate, like those of the green pulp within, by their greater or less turgidity open or close the orifice as the moisture or dryness varies.
444. In the White Lily the stomata are so remarkably large that they may be seen by a simple microscope of moderate power, and may be discerned even by a good hand lens. There are about 60,000 of them to the square inch of the epidermis of the lower face of this Lily-leaf, and only about 3000 to the same space on the upper face. ”

“ While, as I have said, the mechanical cause of the split which constitutes the orifice of the stoma is not quite clear, yet there can be little or no doubt that it, like the rest of the phenomenon, is related to surface tension. It might well be that it is directly due to the presence underneath this portion of epidermis of the hollow air-space which the stoma is apparently developed “for the purpose” of communicating with ”

“ 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. ”

“ In the higher plants the guard-cells of the stomata are crescent-shaped and occur in pairs, the concave sides of the cells facing each other with the aperture between, while in mosses the stomata possesses but a single annular guard-cell which surrounds the aperture. The guard-cells of stomata usually contain chlorophyll-bodies in addition to the ordinary protoplasm. They have the power of increasing or diminishing the size of the aperture under the influence of light and moisture, thus regulating the amount of evaporation from the internal tissues of the plant. ”

“ When a leaf is actively at work making new organic compounds, the stomates are usually open; when unfavourable conditions arise, they are usually closed. They also commonly close at night, when growth (or the utilizing of the new materials) is most likely to be active. It is sometimes safer to fumigate greenhouses and window gardens at night, for the noxious vapours are less likely to enter the leaf. Dust may clog or cover the stomates. Rains benefit plants by washing the leaves as well as by providing moisture to the roots. ”

“ C. Cross-section of white pine leaf (Pinus strobus, L.) . 1, Epidermal and hypodermal cells; 2, Guard cells below the leaf surface; 3, Surrounding cells; 4, Air space below the guard cells; 5, Parenchyma cells with projecting inner walls.
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The relation of the stoma to surrounding cells is best shown in cross-sections of the leaf. In powders the relationship of the stoma to the surrounding cells is, however, readily ascertained. If the guard cells come in focus first, they are above the surface ”

“ Our magnifying power is greater, and the cells appear larger; moreover, we have before us a few cells altogether unlike any of their neighbors, little button-hole-like structures. These are the stomata of the leaf, and through these tiny mouths for the stomata are real openings through the epidermis the exchange of gases goes on between the growing plant and the surrounding atmosphere. ”

“ The stamens are subject to various changes in prolified flowers; they assume, for instance, a leaf-like or petal-like condition, or take on them more or less of a carpellary form, or they may be entirely absent; but none of these changes seem to be at all necessarily connected with the proliferous state of the flower. Of more interest is the alteration in the position of these organs which sometimes necessarily accrues from the elongation of the axis and the disjunction of the calyx; thus, in proliferous roses the stamens become strictly hypogynous, instead of remaining perigynous. ”

“ Leaves dark green, slender, 1′—11⁄2′ long, marked by 6—10 rows of stomata on each face, mostly persistent 4—6 years. Flowers orange-red: male in short crowded spikes; female clustered or in pairs on stout stalks. ”

“ See also succeeding paragraphs and sections in Chloranthy, Virescence, &c.
Phyllody of the stamens happens less frequently than the corresponding condition in the neighbouring organs. The structure of the anther is so much removed from that of the leaf, that the change of the stamen from its ordinary condition to that of a leaf must be regarded as indicating a greater degree of perverted development than that which occurs in those cases where less highly differentiated organs, such as the sepals, petals, and pistils, are thus altered. ”

“ Stems slender, rather stiff, evergreen, 2-4 high, with the stomata arranged in regular rows, rough, 8-34-furrowed, the ridges with two indistinct lines of tubercles, the central cavity large, from one-half to two-thirds the diameter; sheaths rather long, cylindric, marked with one or two black girdles, their ridges obscurely 4-carinate; teeth brown, membranous, soon deciduous; spikes pointed; stem rarely producing branches which are usually short and occasionally fertile; forms are sometimes found with longer sterile branches. ”

“ Another very beautiful device for protecting the underside of the leaf, and one which may be studied in many of our commonest plants, consists of the inrolling of the edges, often combined with a wrinkling or ridging of the underside, so that the stomata are set in deep hollows, communicating with{39} the open air only through narrow openings. ”