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chyma, but are in contact with an internal deep brownishred gland, the lower side of which sometimes appears to have six regular plane faces obliquely resting upon a central face, or, in other cases, to be composed of six cells surrounding a seventh, all being filled with dark red colouring matter. The nature and use of these glands, and of the stomates that accompany them, is unknown. This is I believe the only case hitherto noticed, where the same species has stomates of different forms; it is also remarkable, because in one of these cases the stomate does not open into a chamber of the parenchyma, but immediately reposes upon a gland.
Although the usual condition of stomates is such as is above described, yet there are cases in which it is materially modified, and their function is changed. An instance of this occurs in Dionæa muscipula, in which the peculiar glands, placed in great numbers on the upper side of the lamina of the leaf, each proceed from a pair of parallel green cells, apparently of the same nature as the two cells forming the sphincter of a stomate.
In the epidermis of certain plants are openings resembling stomates, which require to be distinguished from them. In Nuphar luteum they occur in the form of circular depressions (figs. a and b), the sides of which are marked by elevated rings. In Peperomia pereskiæfolia (fig. e) they are deep impressions in the epidermis, at the bottom of which is a two-celled hair. These have been taken for stomates by Meyen, in a plant called by him Pleurothallis ruscifolia (Wiegman's Arch. 1837. t. 10. figg. 4, 5, 6, 7, 8, 9.); but according to Schleiden, the observations of this anatomist are incorrect, and all such appearances are either spaces left by the fall of hairs, whose bases fitted into the cavity, or formed for the reception of hairs, or depressions of entirely a different nature from stomates.
Stomates are not found in Mosses, Fungi, Algæ, or Lichens (see Introduction to the Natural System); in no submersed plants, or submersed plants of amphibious plants. They are not formed in the cuticle of plants growing in darkness, nor upon roots, nor the ribs of leaves. It frequently happens that they are found upon one surface of a leaf, but not on another, and
generally in most abundance on the under side. In succulent parts they are neither rare nor wholly wanting, as has been often asserted; but are, on the contrary, as numerous as on many other parts. They may be generally seen upon the calyx; often on the corolla; and rarely, but sometimes, upon the filaments, anthers, and styles. In fruit, they have only been noticed upon such as are membranous, and not upon the coat of the seed; not even upon those seeds which, as in Leontice thalictroides, grow exposed to air; with the exception of the genus Canna, in which Dr. Schleiden has found them, and to which he thinks them necessary in order to facilitate the passage of fluid through them to the interior of the seed. They exist upon the surface of cotyledons.
Brown thinks that the uniformity of the stomates, in figure, position, and size, with respect to the meshes of the epidermis, is often such as to indicate the limits, and sometimes the affinities, of genera, and of their natural sections. He has shown, with his usual skill, that this is the case in Proteaceæ, in which statement he is supported by Schleiden, who seems to think that the structure of the stomatic opening will be modified according to the physiological peculiarities of particular species, and that it will often indicate affinity. He mentions Cactaceæ, Coniferæ, Piperaceæ, Agave, with some allied Liliaceæ, Commelinacea, and Grasses, in illustration of this. (Wiegm. Arch. 1838. p. 59.) Brown also remarks, that on the microscopic character of the equal existence of stomates on both surfaces of the leaf depends that want of lustre which is so remarkable in the forests of New Holland. (Journal of the Royal Geogr. Society, i. 21.)
The same botanist is of opinion, that the two glands, or rather bladders, of which a stomate is composed, are each analogous to the single bladders often found occupying the inner face of the meshes of the epidermis. (Plate III. fig. 9.) (See the Memoir on the impregnation of Orchidea.) This idea is confirmed by the structure of Yucca (Plate III. fig. 10.), in which the four oblong vesicles surrounding the stomate are evidently of the same nature as the free spheroidal vesicles (cytoblasts?) contained in the cells of the cuticle.
The following table of the proportion of stomates on the
surface of various organs will serve to give some idea of their relative abundance. The first twenty-eight cases are taken from Thomson's Treatise on Vegetable Physiology, in the Library of Useful Knowledge. For the remainder I am answerable:
Number of stomates on one inch
Names of the plants on the leaves of
1 Andromeda speciosa
3 Alisma Plantago
4 Amaryllis Josephinæ
6 Dianthus Caryophyllus 7 Daphne Mezereum
8 Epidendrum .
10 Hydrangea quercifolia 11 Gærtnera.
16 Pittosporum Tobira
17 Philadelphus coronarius
19 Sempervivum tectorum
23 Rumex acetosa 24 Theophrasta
25 Tussilago farfara
27 Vitis vinifera
28 Viscum album
20,000 20,000 40,000
32 Stapelia (stem).
37 Cactus speciosissimus (stem)
The origin of stomates has always been very obscure, nor do I know of any one who has explained their origin in any detail except Schleiden, who considers them to be the last cytoblasts which the cuticular tissue forms (see page 19. for his theory of cytoblasts). He considers that in the beginning all the forms of tissue are, in shape, contents, and structure, exactly the same, and that all the modifications of tissue take place later. He supposes that the exterior tissue of a given mass leaves off producing new cells in its interior sooner than that of the exterior, and that consequently epidermis is first completely organised; but in the epidermis some of the cells retain longer than others the property of forming internal cells, and it is when the last pair of cells separate and absorb their parent that the stoma is produced. The cells forming the stomatic sphincter are in their origin exactly the same as the cells of the parenchyma, and they remain so in their functions throughout their whole existence.
Some physiologists, Link for instance, adhere to the opinion that stomates are glands or secreting organs, and not mere passages in the cuticle for the transmission of gaseous matter. Upon this subject I quote the words of Schleiden :
"These two cells (of the stoma) have been designated by the name of glands, but I do not see any reason for this denomination being given to them in preference to any of the other exactly similar cells of parenchyma. From these they
do not differ at all in the abstract, and in their position only apparently, inasmuch as it is a law that only two cells form an intercellular passage, not three or four; examples of which are not uncommon in the interior of plants. They contain, like the surrounding parenchyma, sometimes gum, sometimes globules of mucus (schleim), sometimes starch, these latter substances sometimes colourless, and sometimes coloured by chlorophyll, but always so that their contents are the same as those of the surrounding cells: but never, as I believe, does one find in them peculiar substances which might warrant the name of glands. In the single instance. of Agave lurida I remember having seen a few drops of oil. The diversity of opinions as to whether the stomata be really open, leads to the supposition, of the correctness of which
any one may easily convince themselves, that their remaining open is not at all caused by a constant exterior influence, but very probably depends upon the momentary vitality of the plant, or of the organ, or perhaps only of the surrounding cellular tissue. The substances which are deposited near and upon the stomata are considered by some, with more or less plausibility, as sufficient evidence that these substances cannot be abstracted from the epidermis itself, and then they jump to the conclusion that such substances are secreted by the I have, however, in vain looked for any facts which might make it even probable, that those secretions should arise rather from the evaporation of the so called glandular cells, than from those of the other parenchymatous cells, and more especially from such as border upon the cavities into which the stomata lead; and it appears to me that this assumed function is, in the present state of our knowledge, a mere petitio principii. Let us take the Coniferæ: here I find gum resin on the stomata; if I remove this by etherial oil, the stomata still remain wide open; then I find a cavity (including the two cells of the stomata), and surrounded by cells which contain gum (schleim), some starch and chlorophyll, but no traces of gum resin or turpentine; on the contrary, I find, much deeper down, large turpentine ducts, and conclude now that the fluid turpentine oil escapes from these passages in the form of vapour, and following the intercellular passages, arrives in the cavities, and from here evaporates by means of the stomata into the atmosphere, by which, as follows from its nature, it leaves behind a certain quantity of resin," &c.
The surface of the epidermis is either perfectly smooth, or furnished with numerous processes, consisting of cellular tissue in different states of combination, which may be arranged under the heads of hair, scurf, glands, and prickles. All these originate either directly from the epidermis, or from the cellular substance beneath it; never having any communication with the vascular or ligneous system.
In Nepenthes the cuticle in the inside of the pitchers is pierced by a great number of holes, each of which is closed up by a firm thick disk of small cellular tissue, deep brown in