94. Acicular raphides are phosphate of lime (Quekett); conglomerate, an oxalate of lime (Id.). Others have a different composition.

Raphides are in fact crystals formed by the action of organic acids: phosphoric, oxalic, citric, tartaric, malic, &c., upon such bases as the fluids of plants may happen to contain. Right rhombic crystals are said to be carbonate of lime; octahedral crystals and six-sided prisms to be sulphate of lime.

95. Besides these, SUGAR, GUM, DEXTRINE, TANNIN, CAOUTCHOUC, and various ALKALOIDS form the contents of tissue.

THE EPIDERMIS AND ITS PROCESSES.

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III. OF THE EPIDERMIS AND ITS PROCESSES.

96. The surface of plants is covered by a skin called the EPIDERMIS, consisting of one or more layers of parenchyma, the cells of which are

65

유

tabular or compressed, and in a

firm state of cohesion 65.

97. The spaces seen upon the epidermis, when examined by a microscope, represent these cells.

This is not the less true, because in some kinds of epidermis the boundary of the cells is sinuous; for in such cases we may suppose the epidermis to be made up of cells with a very uneven surface 65 6.

98. It is, therefore, not a peculiar membrane, but a form of cellular tissue.

99. It is spread over all the parts of plants which are exposed to air, except the stigma.

100. It is not found upon parts habitually living under water.

101. It is itself protected by an extremely thin homogeneous pellicle, which covers every part except the openings through the stomates. This membrane is the cuticle, and may be regarded as a superficial secretion, of the same nature as the intercellular substance (15)

65 c 66

Mohl, however, maintains that the cuticle is really nothing more than the thickened external sides of the epidermoidal cells, and that when it is separable, it is only because it can divide from the protoplasm or secondary lining. He, however, admits that these external cells grow outwardly after they have arrived at a certain stage of development, as is indeed manifest from the condition of such hairs as those of Crucifers, which gradually become quite tuberculated on their surface 67.

Fig. 65.-a Chambers formed beneath the stomates of the leaf of the Balsam, by a net-work of dark parenchymatous cells; c the epidermis of the Iris germanica, with its cuticle, the former with its stomates, the latter with its slits; b epidermis of Madder, with its stomates.

102. It is in some cases composed largely of silicate of potash; in other cases it is hardened by a deposit of calcareous matter.

67

103. The use of the epidermis and cuticle is to guard the subjacent paren chyma from the immediate action of air, and so prevent its being dried up. This seems to be proved conclusively by the thickness of the skin in plants exposed to great drought, its thinness in those which inhabit damp places, and its absence from parts which live under water 66 *

a

66 *

66

104. The epidermis is furnished with stomates.

105. STOMATES are oval spaces lying between the sides of the cells, invariably opening into intercellular cavities in the subjacent tissue, and appearing to be bordered by a limb when they are viewed from above 68 b c e.

Fig. 66.-a Epidermis of the leaf of Aloe margaritifera, showing that the thick dark cuticle is the outer side of the cells of the epidermis, which are lined with pitted protoplasm; b that of Cycas revoluta, in which, in Mohl's view, the continuity of the cuticle is owing to its having grown together at its sides; c that of Ephedra distachya, which shows the same appearances as a. From Mohl.

STOMATES, CISTOMES.

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106. This appearance of a limb is owing to the juxtaposition of two or more elastic vesicles, closing up or opening the aperture which they form, according to circumstances; as is manifest when the stomate is divided perpendicularly to the plane of the epidermis 68 a df.

But there are some modifications of this structure which deserve notice. In Marchantia the stomates resemble chimneys, and are built up of four tiers, each tier consisting of four cells. In Myzodendron 69, in which they occur singly on the apex of each tubercle of the stem, they are stated by Dr. Joseph Hooker to be incorporated into a uniform integument, presenting no trace of cellular origin, lying over a chamber 69 a traversed by viscid filaments, and frequently closed up by an opaque mass.

69

107. It has been asserted that stomates are closed by a membrane, but although that statement was erroneous, it now appears that the cuticle

is sometimes extended into the chamber beneath it, in the form of a funnelshaped expansion, open at both ends, and called the CISTOME 70. In some

h

70

cases appendages proceed from the edge of the cistome through the intercellular passages, to neighbouring cistomes, which are thus connected with

each other.

This was first observed by Gasparrini in 1842, and the presence of these organs has been confirmed by Mohl, who, however, denies the existence of a fibrous structure such as Gasparrini's figures represent.

108. Stomates are found abundantly upon leaves, particularly on the lower surface of those organs; occasionally upon all parts that are modifications of the leaves, especially such as are of a leafy texture; and on the stem.

Fig. 69.-Stomates of Myzodendron punctulatum; a section through stomate and epidermis; b the stomate seen from without; c from within.-Dr. Hooker.

Fig. 70.-a Cistomes of Ornithogalum nutans; on the left are seen the connecting appendages; b a cistome in Cereus peruvianus.-Gasparrini.

The following estimate of the number of stomates found on certain leaves, will give an idea of their importance in the vegetable economy :—

109. Stomates have not been found upon the roots, nor on colourless parasitical plants, nor the submersed parts of plants, nor on Thallogens; they are, moreover, rare, or altogether absent, in succulent parts.

110. It frequently happens, that they are so incompletely formed, as to be either altogether incapable of action, or to act in a very imperfect manner; as in succulent plants.

111. The function of stomates is to regulate evaporation and respiration. It has been thought that the former function, in particular, is that for which they are destined; and that the cause of certain parts becoming succulent, is the absence of stomates in sufficient numbers to carry off the watery part of the sap. But some succulent plants have more stomates than ordinary plants, so that this opinion requires reconsideration.

112. FALSE STOMATES 72 are openings in the epidermis, caused by the falling off of deciduous hairs 72 a

113. HAIRS are minute expansions of transparent cellular tissue proceeding from the surface of plants. They are of two kinds, lymphatic and secreting.

114. Lymphatic hairs are formed by vesicles of cellular tissue placed end to end, and not varying much in dimensions.

115. Glandular hairs are also formed by vesicles of cellular tissue placed end to end, but are sensibly distended

at the apex or base into receptacles of fluid.

72

116. Lymphatic hairs are for the absorption of moisture, for the protection of the surface on which they are placed, and for the control of evaporation through the stomates. They always proceed from the veins, while the stomates occupy the interjacent parenchyma.

In Campanula, on the style, are placed hairs called collectors, in great number. They originate below the epidermis, and have the singular property of being retractile, the upper part being capable of drawing back within the other; their use is unknown. Some hairs, moreover, have numerous parallel spiral threads formed in their interior, as in Acanthodium. Others have but one thread, and some are curiously striated.

117. Glandular hairs are receptacles of the fluid peculiar to certain species of plants, such as the fragrant volatile oil of the sweet briar, and the

Fig. 72.-a Hair of Nuphar luteum; b the hole formed where it drops off.