12 HISTOLOGY [VEGETABLE. Mineral matters in cell- walls. Molecu lar struc ture of cell-wall Chloro phyll. best seen when thin slices of the rhizome are placed for a few days in the solution, and then dried and observed under water. The addition of caustic potash causes the colour to disappear, but it reappears on washing away the potash. The walls of vessels become coloured blue- violet t>y addition of hydrochlorate of phenole, and also when use is made of alcoholic solution of cherry-wood and concentrated hydrochloric acid (Holmel s xylophilin reaction). Mineral matters are of ten deposited in cell- walls. Calcium carbonate occurs rarely, calcium oxalate frequently, and silica is the commonest of all. Calcium carbonate forms the cystolithes of Ficus and of the Acanthacecv, crystals or masses of crystals imbedded in the cell-wall but projecting into the cavity although surrounded by the substance of the wall. In corallines and many algae, as also in the C haras, carbonate of lime is abundant in the cell- walls. Calcium oxalate crystals occur in the cell-walls of many plants, in other cases forming small granules. The crystals of calcium carbonate are soluble in acetic acid, while those of oxalate are not, although soluble in dilute nitric and hydrochloric acids. Silica is abundant in the Diatomacece and also in the cells of many of the higher plants (Equi- setum, grasses, beech, &c,). Products of desorganization or degradation of the cell wall occur in the form of gum, gum-resins, or resins, examples of which may be seen in the cherry, gum-arabic, gum- tragacanth, myrrh, &c. Gum-arabic consists of arabin, gum-tragacanth of bassorin, and cherry-gum is a mixture of the two. These substances, when formed, are apparently of no further use to the plant, and are produced by the destruction or desorganization of the cell-walls, as portions of the cell-wall can be distinctly traced when gum-tragacanth is examined microscopically. Cell-walls, as those of the wood of Conifers, bast-cells, and cells of ivory nut, and starch granules, are found when examined by polarized light to be doubly refracting. By an elaborate series of researches Niigeli concluded that these structures were made up of crystalline doubly refract ing particles or micellae, each consisting of numerous atoms and impermeable by water, although each of the micellae is surrounded by a thinner or thicker layer of water. The water may increase or diminish within certain limits with out destroying the structure ; or under certain conditions as by the application of certain reagents (strong acids and alkalies, ammoniacal solution of cupric oxide), the texture can be destroyed by the swelling up of the part. The water between the micellae may be removed by drying, when the micellae themselves come into contact, as the presence of air would destroy the transparency of the membrane. This peculiar molecular composition of the wall at once explains the striation and stratification observed in it, and also enables us to understand growth and nutrition by the intussusception of new particles in the water space between the micellae. 1 Certain substances are formed by the protoplasm and separate from it in the form of granules or crystal-like bodies. The most important of these substances are chlorophyll and starch, the less important are aleurone grains and crystalloids. Chlorophyll or leaf-green is the green colouring matter of plants, and is met with most frequently in the leaves and young stems. The colouring matter is always united with the protoplasm, usually to definite rounded masses, the chlorophyll granules or corpuscles, readily distinguishable, from the general protoplasmic mass of the cell in which they are imbedded. Chlorophyll granules never occur separate from the protoplasm of the cell. In a few instances the whole of the protoplasmic mass, with the exception of the ectoplasm, is uniformly coloured green as in Plcurococcus and other low algae ; while in other plants the protoplasmic base for the colouring matter is star-like (Zygnema), in plates or lamellae (Clostcrium and Meso- ca.rpus), or spiral, as in Spirogyra. The chlorophyll grains of the vast majority of plants are rounded corpuscles of varying size with 1 See Na geli and Schwendener, Das MikrosJco}* (2d ed.), p. 299 sq. ; and Dippel, Do<s AIikrosl;<n>, vol. i. p. 409 sq. a slightly denser external layer, and frequently containing vacuoles or small starch granules. They grow in size and divide, the grain elongating and being cut into two by the formation of a gradually deepening circular groove. These changes may be seen in the pro- thallus of a fern or the leaf of a mosg. The granules are produced by the aggregation of protoplasmic particles, so as to form a sharply- defined spherical mass. At first these are colourless or of a yellow tinge, and become green by the formation of the colouriug matter, the chlorophyll, when exposed to the light, as it is only in a few rare cases, as in the cotyledons of pines and in ferns, that the colour iug matter is formed independently of light. The colouring matter can be removed by means of alcohol, ether, benzole, chloroform, and other solvents, the protoplasmic mass remaining behind unchanged in size and appearance, except in so far that it is now colourless. The solution thus obtained is of a dark green colour by transmitted light, and blood-red by reflected light. Its spectrum shows seven absorption bands, the strongest being between the lines B and c of the solar spectrum. Many modifications of chlorophyll exist in plants, and it also undergoes changes in colour during the ripening of fruits or in the corollas of certain flowers. The chief modifications are etiolin, in blanched parts of plants ; anthoxanthin, in yellow granules of many flowers ; xanthophyll, yellow granules in leaves in autumn ; the green colour ing matter of red sea-weeds ; phycoerythriu, the red colouring matter of red sea- weeds ; the phycochrome of nostoc, &c. ; and the brown colour of diatoms and fucoids. Starch occurs in granules of varying size and form, and during Start! the growth of the granule it is always in relation to the protoplasm of the cell. The granules are oval, lenticular, polyhedral, or bone- shaped, as may be seen in the potato, wheat, and maize, and in the milk-sap of certain exotic Euphorbias respectively. Each grain usually exhibits a central or lateral spot, the hilum, and a series of concentric striae, caused like the striation and stratifica tion of the cell-wall by the alternation of more and less watery layers. Sometimes the starch granule has two or more hila, the compound grains, which often separate into their several parts. Starch has the same chemical composition as cellulose, C (; H ]0 5 , and differs from cellulose in being coloured blue directly by a dilute solution of iodine. Schacht s solution contains 1 grain of iodine and 3 grains of iodide of potassium dissolved in 1 ounce of distilled water ; but an aqueous solution of iodine answers quite well. Two substances are generally recognized in the starch grain (1) granulose, coloured blue by iodine and forming by far the greater part of the granule, and (2) starch cellulose, not coloured blue and only forming a sort of skeleton to the grain. Starch is one of the most widely distributed substances in plants, being absent from comparatively few except the fungi. Oil globules occur not unfrequently in the protoplasm of plants ; and in a few instances they occur in chlorophyll granules. Oil is easily distinguished by its reactions with ether, and by its optical properties. Occasionally portions of the protoplasm assume a crystal-like ap- Crysta pearance, resembling cubes, octohedra, tetrahedra, &c. These por- loids, tions are known as crystalloids or protein crystals. They give the globoii ordinary reactions of protoplasm, and differ from crystals in their and power of swelling up and changing their angles in certain solutions, alcuro: as in caustic potash. Crystalloids occur frequently in the cells of grains, the tuber of the potato, in fatty seeds, in red alga?, in petals of many flowers (Viola tricolor), and in some fruits. Usually the crystalloids occur in fatty seeds, as in the castor oil and brazil nut, in the interior of rounded grains of albuminoids, the aleurone or protein grains, along with little rounded bodies called globoids consisting of a combination of magnesia and lime with phosphoric acid. In other instances aleurone grains without crystalloids are met with, as in Cynoglossum. The aleurone grains are usually soluble in water, and are, therefore, best examined microscopically in strong glycerin, in iodine dissolved in glycerin, or in a solution of corrosive sublimate in alcohol. Aleurone grains form when the seed is nearly ripe, the crystalloids and globoids appearing earlier. The cell-sap consists of water with different substances in Cell-sa solution, the substances varying in different cells, and also changing in the same cell from time to time during growth. It saturates the whole wall and protoplasm, and collects in the vacuoles and cell-sap cavity. The most important substances in the cell-sap are innlin, sugar, tannin, and colouring matters, while the calcium oxalate usually crystallizes out, and forms visible crystals in the cell, or in the wall as already described. Inulin can be separated, in the form of sphserocrystals, by the action of alcohol or glycerin, from the tissues of many of the Composite, dahlia, sunflower, &c. By keeping the tissue long in absolute alcohol the crystals grow to a large size, and occupy more than one cell. Sugar in solution in the, cell-sap may be grape or cano sugar, and can be rendered visible by the copper test, or by the action of glycerin. Glycerin forms drop-like spheres
with sugar and inuliu ; these are very highly refracting and easily
