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HISTORY] stomata have been ruptured and cast off with the rest of the epidermis. Both cork and phelloderm may be differentiated in various ways. The former often has its cells lignified, and may consist of alternate layers of hard and soft cells. The latter may develop stereom, and may also be the seat of origin of new formations of various kinds—e.g. supplementary vascular bundles, anomalous cambial zones, &c. It is often enormously developed and forms a very important tissue in roots. In the stem of a tree the original phellogen is replaced by successive new phellogenic layers of deeper and deeper origin, each forming its own layer of cork. Eventually the new phellogens reach the level of the secondary phloem, and are formed in the parenchyma of the latter, keeping pace in their inward march with the formation of fresh secondary phloem by the cambium. The complex system of dead and dying tissues cut off by these successive periderms, together with the latter themselves—in fact, everything outside the innermost phellogen, constitutes what is often known botanically as the bark of the tree. Rhytidome is, however, a preferable term, as the word bark has long been established in popular usage to mean all the tissue that can easily be peeled off—i.e. everything down to the wood of the tree. The rough surface of the bark of many trees is due to the successive phellogens not arising in regular concentric zones, but forming in arcs which join with the earlier-formed arcs and thus causing the bark to come off in flakes or thick chunks. A layer of cork is regularly formed in most Phanerogams across the base of the petiole before leaf fall, so as to cover the wound caused by the separation of the leaf from the stem. Special “wound-cork” is also often formed round accidental injuries so as to prevent the rotting of the tissues by the soaking in of rain and the entrance of fungal spores and bacteria. A peculiar modification of periderm is rormed by the phellogen in the submerged organs (roots or stems) of many aquatic or marsh-loving plants. This may take various forms and may cover the whole of the organ or be localized in special regions; but its cells are always living and are separated by very large intercellular spaces containing air. This tissue is called aerenchym, and no doubt its function is to facilitate the respiration of the organs on which it is formed and to which the access of oxygen is difficult. In other cases, a similar formation of spongy but dead periderm tissue may occur for the same purpose in special patches, called pneumatodes, on the roots of certain trees living in marshy places, which rise above the soil in order to obtain air.

History and Bibliography.-The study of plant anatomy was begun in the middle of the seventeenth century as a direct result of the construction of microscopes, with which a clear view of the structure of plant tissues could be obtained. The Englishman Grew and the Italian Malpighi almost simultaneously published illustrated works on the subject, in which they described, for the most part very accurately, what they saw with the new instruments. The subject was practically dormant for nearly a century and a half, largely owing to the dominance of classificatory botany under the influence of Linnaeus. It was revived by several German workers, prominent among whom were Treviranus and Link, and later Moldenhawer, as well as by the Frenchmen Mirbel, at the beginning of the 19th century. The new work largely centred round a discussion of the nature and origin of vessels, conspicuous features in young plant tissues which thus acquired an importance in the contemporary literature out of proportion to their real significance in the construction of the vascular plant. The whole of the writings of this time are dominated by a preoccupation with the functions of the different tissues, in itself an excellent standpoint for investigation, but frequently leading in the case of these early investigators to one-sided and distorted views of the facts of structure. The pioneer of modern plant anatomy was Hugo von Mohl (fl. 1840), who carefully investigated and

described the facts of anatomical structure without attempting to fit them into preconceived views of their meaning. He produced a solid body of accurately described facts which has formed the secure groundwork of subsequent advance. From Mohl down to the eighth decade of the century the study of anatomy was entirely in the hands of a group of German investigators, prominent among whom were several of the most eminent founders of modern scientific botany—such, for instance, as Nageli, Sanio and De Bary. To the first we owe the secure foundation of our knowledge of the structure and course of the vascular strands of the higher plants (“Ueber den Bau und die Anordnung der Gefässbündel bei den Stamm und Wurzel der Phanerogamen, ” Beiträge zur wissenschaftlichen Botanik, Heft i., Leipzig, 1859); to the second the establishment of the sound morphological doctrine of the central cylinder of the axis as the starting-point for the consideration of the general arrangement of the tissues, and the first clear distinction between primary and secondary tissues (Botanische Zeitung, 1861 and 1863); to the last the putting together of the facts of plant anatomy known up to the middle of the eighth decade of the century in that great encyclopedia of plant anatomy, the Vergleichende Anatomie der Vegetationsorgane bei den Phanerogamen und Farnen (Stuttgart, 1876; Eng. trans., Comparative Anatomy of the Vegetative Organs of the Phanerogams and Ferns, Oxford, 1882). In 1870-1871 Van Tieghem published his great work, “Sur la Racine,” ''Ann. sci. nat. bot.'' (Paris). This was not only in itself an important contribution to plant anatomy, but served as the starting-point of a series of researches by Van Tieghem and his pupils, which has considerably advanced our knowledge of the details of histology, and also culminated in the foundation of the doctrine of the stele (Van Tieghem and Douliot, “Sur la polystélie,” Ann. sci. nat. bot., 1887; Van Tieghem, Traité de botanique (2nd ed. Paris, 1889-1891). This has had a most important effect on the development in recent years of morphological anatomy.

In the progress of the last three decades, since the publication of De Bary's great work, five or six main lines of advance can be

distinguished. First, the knowledge of the details of histology has of course advanced greatly in the direction through the ceaseless activity of very numerous, mainly German, workers, though no fundamentally new types of tissue have been discovered. Secondly, the histology of fossil plants, particularly woody plants of the carboniferous period, has been placed on a sound basis, assimilated with general histological doctrine, and has considerably enlarged our conceptions of plant anatomy as a whole, though again without revealing any entirely new types of structure. This branch of the subject, founded by Corda, Göppert, Stenzel and others in Germany, was enormously advanced by Williamson's work on the Coal Measures plants, recorded in the magnificent series of memoirs, “Researches on the Organization of Fossil Plants of the Coal Measures” (Phil. Trans. Roy. Soc., vols. i.-xix., 1871-1893). The work of Solms Laubach in Germany, Renault and Bertrand in France, and in recent years, of Zeiller in France, and Scott, Seward and others in England, has advanced our knowledge of the anatomy of fossil plants in an important degree. While convincing us that the plants of past ages in the earth's history were exposed to very similar conditions of life, and made very much the same adaptive responses as their modern representatives, one of the main results of this line of work has been to reveal important data enabling us to fill various gaps in our morphological knowledge and to obtain a more complete picture of the evolution of tissues in the vascular plants. One of the most striking incidents in the progress has been the recognition within the last few years of the existence of an extinct group of plants lying on the borderland between Filicales and Gymnosperms, and known as the Cycadofilices, a group in which, curiously enough, the reproductive organs remained undiscovered for some time after the anatomy of the vegetative organs was sufficiently well known to afford clear evidence of their true affinities. Thirdly, we have to record very considerable