If we examine a portion of the layers of an onion ([Plate XIV]., No. 3), or a thin section of the stem or root of the garden rhubarb (No. 4), we shall find many cells in which either bundles of needle-shaped crystals or masses of a stellate form occur, not strictly raphides.
Raphides were first noticed by Malpighi in Opuntia, and subsequently described by Jurine and Raspail. According to the latter observer, the needle-shaped or acicular are composed of phosphate, and the stellate of oxalate of lime. There are others having lime as a basis, in combination with tartaric, malic, and citric acids, all of which are destroyed by acetic acid; others are soluble in many of the fluids employed in mounting. These crystals vary in size from the 1⁄40th of an inch, while others are as small as the 1⁄1000th. They occur in all parts of the plant; in the stem, bark, leaf, petals, fruit, root, and even in the pollen, and occasionally in the interior of cells. In certain species of aloe, as Aloe verrucosa, we are able to discern small silky filaments; these are bundles of the acicular form of raphides, and probably, as in sponges, act as a skeleton support to the internal soft pulpy mass.
PLATE XIV.
STELLATE AND CRYSTALLINE TISSUE OF PLANTS.
In portions of the cuticle of the medicinal squill (Scilla maritima) large cells are found full of needle-shaped crystals. These cells, however, do not lie in the same plane as the smaller cells of the cuticle. In the cuticle of an onion every cell is occupied either by an octahedral or a prismatic crystal of calcium oxalate. In some specimens the octahedral form predominates; in others, even from the same plant, the crystals are prismatic and arranged in a stellate form, as in that of the grass (Pharus cristatus). ([Plate XIV]., No. 6.)
Raphides of peculiar figure are found in the bark of certain trees. In the hickory (Carya alba) may be observed masses of flattened prisms having both extremities pointed. In vertical sections from the stem of Elæagnus angustifolia, numerous raphides of large size are embedded in the pith, and also found in the bark of the apple-tree, and in elm seeds, every cell containing two or more minute crystals.
In the Graminaceæ, especially the canes; in the Equisetum hyemale, or Dutch rush; in the husk of rice, wheat, and other grains, silica in some form or other is abundant. Some have beautifully-arranged masses of silica with raphides. The leaves of Deutzia scabia, No. 7, are remarkable for their stellate hairs, developed from the cuticle of both their upper and under surfaces; forming most interesting and attractive objects examined under polarised light. ([Plate VIII]., No. 173.)
Silica is found in the structure of Rubiaceæ both in the stem and leaves, and, if present in sufficient thickness, depolarises light. This is especially the case in the glandular hairs on the margins of the leaves. One of the order Compositæ, a plant popularly known as the “sneezewort” (Archillæ ptarmica), has a large amount of silica in the hairs found about the serratures of its leaves.
All plants are provided with hairs; some few with hairs of a defensive character. Those in the Urtica dioica, commonly called the Stinging-nettle, are glandular hairs, developed from the cuticle, and contain an irritating fluid; in other hairs a circulation is visible: examined under a power of 100 diameters, they present the appearance seen at [Plate XIII]., No. 19.