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 OF PLANTS conspicuous features in plant tissues which acquired an importintended to replace the epidermis as the external protective layer ance quite out of proportion to their real place in the .construction of the plant when the latter, incapable as it is of further grow th of the vascular plant. The whole of the writings of this time after its original formation, is broken and cast off by the increase are dominated by a preoccupation with the functions of the difin thickness of the stem through the activity of the cambium. ferent tissues, in itself an excellent standpoint for investigation, Cork is also formed similarly in the root after the latter has passed but frequently leading in the case of these early investigators through its primary stage as an absorptive organ, and has its struc- to one-sided and distorted views of the facts of structure. The ture assimilated to that of the stem. The internal tissue foimed pioneer of modern plant anatomy was Hugo von Mohl (fl. 1840), by the phellogen is known as phelloderm, and consists usually of who carefully investigated and described the facts of anatomical ordinary parenchyma. The phellogen may arise, m the first place^ structure without attempting to fit them into preconceived views in any tissue of the axis external to the actual vascular tissues their meaning. He produced a solid body of accurately dei e in the epidermis itself (rarely), m any layer of the cortex, of scribed facts which has formed the secure groundwork of subor in the pericycle. Its most usual seat of origin in the stem is sequent advance. From Mohl down to the eighth decade of the external layer of the cortex immediately below the epidermis, the century the study of anatomy was entirely in the. hands in the root, the pericycle. All the tissues external to the cork of a group of German investigators, prominent in which are are cast off by the plant. The extent of development of the names of several of the most eminent founders of modern phelloderm is dependent upon whether the phellogen has a super- the scientific botany—such, for instance, as Nageli, Sanio, and De ficial or a deep-seated origin. In the former case the formation of Bary. To the first we owe the secure foundation of our knowledge phelloderm is trivial in amount; in the latter, considerable, since of the structure and course of the vascular strands of the higher this tissue has to replace the cast-off cortex, as a metabolic and (“Ueber den Bau und die Anordnung der Gefassbiindel bei particularly a storage tissue. Provision is made for gaseous plants Stamm undWurzel der Phanerogamen, ” Beitriige zur Wisseninterchange between the internal tissues and the external air den schaftlichen Botanik, Heft i., Leipzig, 1859) ; to the second the after the formation of cork, by the development of lenticels. establishment of the sound morphological doctrine of the central These are special organs which interrupt the continuity of tfie cylinder of the axis as the starting-point for the consideration of impermeable layer of ordinary cork-cells. A lenticel is formed, by the general arrangement of the tissues, and the first clear distincthe phellogen at a given spot dividing very actively and giving tion°between primary and secondary tissues (Botanische Zeitung, rise to a loose tissue of rounded cells which soon lose their con- 1861 and 1863) ; to the last the putting together of the facts of tents, and between which air can pass to the tissues below. A anatomy known up to the middle of the eighth decade of lenticel appears to the naked eye as a rounded or elongated scar, plant century in that colossal encyclopsedia of plant anatomy, the often forming a distinct prominence on the surface of the organ. the Vergleichende Anatomie der Vegctationsorgane bei den PhaneroThe lenticels of the stem are usually formed beneath stomata, gamen und Farnen, Stuttgart, 1876. (English translation : Comwhose function they take up after the stomata have been ruptured parative Anatomy of the Vegetative Organs of the Phanerogams and and cast off with the rest of the epidermis. Both cork and Ferns, Oxford, 1882.) In 1870-71, Van Tieghem published his phelloderm may be differentiated in various ways. Ihe former work, “Sur la Racine,” Ann. Sci. Hat. Bot. Paris. This often has its cells lignified, and may consist of alternate layers of great was not only in itself an important contribution to plant anatomy, hard and soft cells. The latter may develop stereom, and may but served as the starting-point of a series of researches by Y an also be the seat of origin of new formations of various kinds—e.^., Tieghem his pupils, which have considerably advanced our supplementary vascular bundles, anomalous cambial zones, etc. knowledgeand the details of histology, and also culminated in the It is often enormously developed and forms a very important foundation of of the doctrine of the stele (Van Tieghem and Douliot, tissue in roots. In the stem of a tree the original phellogen is “Sur la Polystelie,” Sci. Hat. Bot. 1887 ; Van Tieghem, replaced by successive new phellogenic layers of deeper and deeper TraiU de Botanique, Ann. ed. Paris, 1889-91), which has had a origin, each forming its own layer of cork. Eventually the new most important effect 2nd the development in recent years of phellogens reach the level of the secondary phloem, and are formed morphological anatomy.on In the progress of the last quarter in the parenchyma of the latter, keeping pace in their inward of a century, since the publication De Bary’s great work, march with the formation of fresh secondary phloem by the cam- four main lines of advance can be of First, the bium. The complex system of dead and dying tissues cut off by knowledge of the details of histologydistinguished. has of course advanced these successive periderms, together with the latter themselves in every direction through the ceaseless activity of in fact, everything outside the innermost phellogen, constitutes greatly numerous, but mainly German, workers, though no fundawhat is often known botanically as the bark of the tree. Rhy- very new types of tissue have been discovered. Secondly, tidome is, however, a preferable term, as the word bark has long mentally histology of fossil plants, particularly woody plants of the been established in popular usage to mean all the tissue that can the period, has been placed on a sound basis, aseasily be peeled off—i.e., everything down to the wood of the tree. carboniferous The rough surface of the bark of many trees is due to the succes- similated with general histological doctrine, and has considerenlarged our conceptions of plant anatomy as a whole, sive phellogens not arising in regular concentric zones, but forming ably again without revealing any entirely new types of strucin arcs which join with the earlier-formed arcs, and thus causing though the bark to come off in flakes or thick chunks. A layer of cork ture. This branch of the subject, founded by Corda, Goppert, and others in Germany, was enormously advanced by is regularly formed in most Phanerogams across the base of the Stenzel petiole before leaf fall, so as to cover the wound caused by the Williamson’s work on the Coal Measures plants, recorded in the separation of the leaf from the stem. Special “wound-cork is magnificent series of memoirs, “Researches on the Organization also often formed round accidental injuries so as to prevent the of Fossil Plants of the Coal Measures,” Phil. Trans. Roy. Soc. 1871-93. The workof Solms Laubachin Germany, Renault rotting of the tissues by the soaking in ot rain and the entrance i.-xix., Bertrand in France, and in quite recent years, of Zeiller of fungal spores and bacteria. A peculiar modification of peri- and in France, and Scott and Seward in England, has advanced our derm is formed by the phellogen in the submerged organs (roots or stems) of many aquatic or marsh-loving plants. This may knowledge of the anatomy of fossil plants in an important degree. take various forms and may cover the wholff of the organ or be While convincing us that the plants of past ages in the earth’s localized in special regions ; but its cells are always living and history were exposed to very similar conditions of life, and made .are separated by very large intercellular spaces. This tissue is very much the same adaptive responses as their modern reprecalled aerenchym, and no doubt its function is to facilitate the sentatives, one of the main results of this line of voik has been respiration of the organs on which it is formed and to which the to reveal important data enabling us to fill various gaps in our access of oxygen is difficult. In other cases, a similar formation morphological knowledge and to obtain a more complete picture of spongy but dead periderm cells may occur for the same purpose of the evolution of tissues in the vascular plants. One of the in special patches, called pneumatodes, on the roots of certain most striking incidents in the progress has been the recognition trees living in marshy places, which rise above the soil in order within the last few years of the existence of an extinct group of plants lying on the borderland between Ferns and Gymnosperms, to obtain air. History and Bibliography.—The study of plant anatomy was and known as the Cycadofilices, a group in which, curiously begun in the middle of the seventeenth century as a direct result enough, the reproductive organs have not yet been discovered, but of the construction of microscopes, with which a clear view of the the anatomy of whose members affords sufficient evidence of their , structure of plant tissues could be obtained. The Englishman true affinities. Thirdly, we have to record very considerable Grew and the Italian Malpighi almost simultaneously published progress in our knowledge of that distinctively morphological illustrated works on the subject, in which they described, for the anatomy to which reference was made in the introduction to this most part very accurately, what they saw with the new instru- article. The Russian plant-anatomist, Russow, may be said to ments. The subject was practically dormant for nearly a century have founded the consideration of plant tissues from the point ot and a half, largely owing to the dominance of classificatory botany view of descent (Vergleichende Untersuchungen iiber die Leitbiindelunder the influence of Linnaeus. It was revived by several German kryptogamen. St Petersburg, 1872 ; and Betrachtungen uber workers, prominent among whom were Treviranus and Link, and Leitbundel und Grundgewebe. Dorpat, 1875). He has been ably later Moldenhawer, as well as by the Frenchman Mirbel, at the followed by Strasburger {Ueber den Bau und die Verrichtungen bemnning of the nineteenth century. The new work largely der Leitungsbahnen in den Pflanzen, Jena, 1891), Haberlandt centred round a discussion of the nature and origin of vessels, and others. The explicit adoption of this point of view has had 418

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