HISTOLOGY. 105 HISTOLOGY. to water) and thickened in spiral or annular lines or in reticulate patterns (Fig. 1). At maturity the protoplasm disappears, leaving (jiily the cell- wall of service to the jjlants. Tracliea- are simi- lar to tracheids in the sculpturing of their walls, but instead of being single cells they are formed by the fusion of a row of cells lying originally end to end, the end walls being resorbcd as the lateral walls thicken and the protoplasm disap- pears. At maturity the long empty tubes thus formed show little trace of the cells from which they originated. In angiospcrms they constitute the greater part of the xylem bundles, changing to tracheids as the bundles grow smaller and come to an end (Fig. 7b). But in gyninosperms (pines and their allies) trachcip .are formed only ill the primary xylem, almost all the secondary xylem being tracheids with characteristic circu- lar-bordered pits (Fig. 7a). Tracheie and tra- cheids are the most efficient tissues for the trans- port of water in the larger plants. (5) Sieve tubes are cell fusions formed by the partial resorption of the end walls of a row of young cells. The end partitions and sometimes tlie lateral walls which adjoin other sieve tubes become perforated, forming a so-called 'sieve Fig. 8. 8IETE TUBE AND CO.MPANION CELL. 1, In longitudinal section; 2, in transverse section show- lii|^ a sieve plate. plate,' through which the contents of the sieve tubes (a slimy mi.xture of soluble proteids, car- bohydrates, and other foods) pass freely. Sieve tubes are found in the phloem bundles, in which they constitute the most efficient tissue for the transport of foods (Fig. 8). ((!) Latex tubes are long, much-branched tubes, with free or anastomosing branches, which con- tain the milky or colored sap in certain plants. There are two sorts, articulated and non-articu- lated. A non-articulated tube arises by early dilTerentiation of certain cells in the embryo, which push their way among the other developing tissues by independent growth. The articulated tubes arise by the fusion of rather indefinite rows of cells, either longitudinal or transverse, which form a network of irregular tubes. Latex tubes are found in the phloem bundles, or just outside these bundles in the cortex of stems, and accom- pany them into the leaves, where they ramify widely. Their terminals come into close rela- tion with the nutritive cells. (See illustration under Latex.) They seem to constitute a trans- portation system for foods. See Growth ( in Plants) ; Anatomy of Plants; Cell (in Plants); Root; Stem; Conduction; Latex. HISTOLOGICAL TECHNIQUE. The methods of histological observation were at first extremely crude, consisting merely in tearing apart the tissues and examining them under the microscope. Such handling, of course, largelj' destroyed the relations of the ditTcrent elements of the tissues to one another. The cut- ting of thin sections of tissue with a razor was soon introduced, this being much facilitated by the previous hardening of the tissues in some suit- able solution. Such sections were, however, diffi- cult to study, owing to their transparency and the fact that the ditTerent tissue elements possess nearly the same index of refraction. The -staining of sections was the next great improvement in technique. At first a single stain was used, the denser elements taking a darker shade than those of less dense structure. The discovery of what is known as differential staining, whereby the dif- ferent tissue elements are stained different colors, and the introduction of an instrument for section cutting, known as the microtome (see further below), together with improvements in the micro- scope, have been the main factors in the recent rapid development of the art. At present the most commonly used technical procedures in the examination of tissues and organs is as follows: The first step is fixation. By this is meant a rapid killing of the tissue in such a way a.s to allow it to retain as nearly as possible the same form and relation as in the living tissue. Among the more common fixatives may be mentioned alcohol of various strengths, formalin in from 5 per cent, to 20 per cent, solutions, osmic acid, usually in 1 per cent, solution, MiiUer's fluid (2.5 grams of potassium bichromate and 1 gram of sodium sulphate, dissolved in 100 cubic centi- meters of water), Fleming's fiuid (a mixture of osmic, chromic, and acetic acids in water) , chrom- acetic acid (1 per cent, solution in water), and corrosive sublimate in saturated aqueous solu- tion. Many other fixatives are used for special purposes ; thus, osmic acid is employed for the demonstration of fat and myelin. The small slices of tissue employed are allowed to remain from 12 to 48 hours in a large amount of fixative. Fixed tissues which have been fixed by any other agent than alcohol are then subjected to prolonged washing with running water. The next step is hardeninjj. The fixing agents also act as harden- ing agents if allowed to act sufficiently long. Many fixatives, however, have a detrimental ef- fect upon the tissues if their action is too pro- longed. It is. therefore, quite common to trans- fer the tissues after proper fixation to some new lluid for the purpose of hardening and preserva- tion. The almost universal hardening and pre- serving agent is alcohol, but formalin, too, in from 5 per cent, to 10 per cent, aqueous solution, is now extensively used. The first alcohol bath for plant tissues should be .S5 per cent, or weaker, while many animal tissues will stand 50 per cent. This should be followed consecutively by 50 per cent.. 70 jier cent., 00 per cent., and 97 fier cent, alcohol. For long preservation SO per cent, alcohol is the most satisfactory. Next in the process is imhcddinn. By this is meant the impregnation of the tissues with a liquid which afterwards hardens, thus holding the tissues in
