1. Laticiferous Tissue; 2. Vertical section of a Leaf of the India-rubber Tree, with a central gland; 3. Vertical cast of spiral tubes of Opuntia.
For the study of sieve-tubes take the vegetable marrow, in which they are of extraordinary size. Cut transverse sections of the stem and stain with eosin, and mount them in glycerine. The general arrangement will be seen to differ from that of most other herbaceous plants. Below the epidermis a thick walled band of sclerenchyma with lignified walls will be seen distinct from the vascular bundles, which readily take a stain. The vascular bundles are separate and distinct, and the structure of the bundle is abnormal, there being in each a separate central mass of xylem, with the phloem masses lying, the one central, the other in the peripheral side. Between the xylem and the phloem masses is the cambium layer. The structure being the same in both will serve for the study of the punctate sieve-plates; these are readily stained with eosin, as shown in Sach’s text-book.
Laticiferous Tissues ([Fig. 318]).—In cutting sections of latex care must be taken to at once transfer them to alcohol so as to prevent the flow of the latex from the cells, otherwise the laticiferous vessels will be much less easily traced. The better method is to plunge the root of the dandelion (Leontodon taraxacum), after cleaning, into alcohol, and there let it remain until it has become hardened; then cut thin tangential sections from the phloem, and longitudinal sections through the cambium, and mount them in potash and glycerine. The laticiferous vessels appear circular in the transverse sections with brown contents; these are distributed in groups round the central xylem. Observe in such sections the presence of sphere crystals of inulin. These are formed quite irrespective of the cell-walls.
Laticiferous cells are readily seen in the cortex of Euphorbia splendens, cut just outside the vascular ring. Long tubes will be seen to run through the cortical parenchyma, with thick cellulose walls and granular contents. These are the laticiferous cells, the branching of which distinguishes them from the preceding structure. Included in the granular contents are starch grains of a peculiar dumb-bell form.
Leaf or Petiole.—The general morphology of leaf tissue is essentially the same as that of the stem from which it proceeds. In the typical monostotic stem of Phanerogamæ each leaf receives a portion of the stele or central cylinder of the stem. Such portion is termed a meristele, and may be either entire or split up into a number of schizosteles.
The microscopical structure of leaves should be studied in the whole organ, and by the aid of isolating elements. The whole or portion of a leaf should be soaked in chloral hydrate solution; this will render it transparent, whereby the internal structure can be studied as a whole. Sections should be prepared from fresh leaves, or dried ones softened by soaking in water. Cut them transversely, both in the direction of the mid-rib and at right angles to it. This is best effected by placing the material between two pieces of elder pith or fresh carrot. Sections of the whole are made and transferred to a dish of water. Leaf sections are easily made for examination by macerating the leaves in solution of caustic potash varying in strength from one to five per cent. The epidermis on both sides may be detached, and the elements of the mesophyll and vascular bundles isolated for separate examination.
Potassium permanganate proves to be a useful reagent. A weak solution causes the protoplasmic structures to swell up, thus assisting in the observation of the structure of the chromatophores. This solution may also be employed as a macerating fluid. Beautiful preparations are obtained in this way of the sieve-tubes of Vitis.
Special structural peculiarities are to be observed in the leaves of various plants in which the epidermis consists of more than a single layer of cells (e.g., the leaves of Ficus, Peperaceæ, Begoniaceæ, &c.), cystoleths in the cells of the epidermis of Urtica; glandular structure in Ruta, Psorales; the coriaceous leaves of the Cherry Laurel, and the cylindrical leaves of Stonecrop (Sedum acre).
Reproductive Organs.—The development of the rudiments of flowers is of an extremely interesting nature, and the complete flower should be carefully studied. Median sections are best suited for the purpose. In the large majority of plants the calyx is developed first, then the corolla, and next the stamens. Preparations should be made from materials hardened in alcohol, or first fixed with a strong solution of picric acid and then hardened in alcohol.
Pollen-grains.—Microspores are found lying free in sections made of the reproductive organs; these may be transferred to a glycerine fluid and examined under a high power. They are mostly spherical, with granular protoplasmic contents, in which with much difficulty two nuclei can be made out. Mount and examine, as types of the various forms of granules, the pollen of Helianthus, Althœa, Cucurbita, Ænothera, Orchis, Mimosa, Tulipa, &c. Mount any of these pollen-grains in a weak solution of cane-sugar (about five per cent.), examine with a high power, and note the configuration of their walls with a medium power under polarised light. If transverse sections be made from very young buds, the development of the anther and the pollen may be traced. The material should be preserved in strong alcohol, and the sections treated with equal parts of alcohol and glycerine, and exposed in a watch-glass that the alcohol may evaporate. By this method sections may be prepared for illustrating the formation of the tapetum, special mother-cells, and division of the nucleus.