Cork and Bark Structures

come to the consideration of cork and bark. These formations, though they may form layers several inches in thickness, are nevertheless physiologically related to the cuticle, which is frequently immeasurably thin. Cork and Bark. While, as above stated, the epidermis consists of a single layer of cells, the bark-covering, as a rule, consists of several or many layers of cells. Cork several layers in thickness may result from simple cnticularization of ordinary parenchyma- cells, but in the majority of cases cork is the result of a special process of cell-division. This process of cell-division has the greatest similarity to " cambial activity," that is, to the cell-forming process in the ring between the wood and bark (cambium -ring) of our trees. In the case first mentioned the cork-cells do not necessarily lie in radial series, while in the latter case this arrangement is charac- teristic. The cork-cambium (phellogen), as well as the abovementioned cambium between wood and bark, is, as a rule, a bipolar formative tissue. In only a few cases it is one-sided, that is, forms cells which become cuticularized from without inward. Ordinarily in bipolar cork-cambium activity the numerous outer cells become cuticularized centripetally. There are formed inwardly less numer- 1 The same investigator observed a shrub (Reaumuria Mrtella) in the Arabian desert in which epidermal glands secrete a hygroscopic saline substance which absorbs moisture from the air during the night. TISSUES AND SIMPLE ORGANS. 57 ous cells of the character of primary parenchyma called " phel- loderm," or " cork-parenchyma cells." The various layers formed outwardly are not all equal : there may be alternate layers with thick and thin cell-walls (Betula alba). The well-known " peeling" or "scaling" of bark will occur very readily along the thin-walled layers, because they are only slightly extensible as compared with the thick-walled layers, in which cellulose predominates. The thin-walled layers consist essentially of suberin, a fatty substance, which, besides other con- stituents, contains stearin (v. HOHNEL, KUGLER). The microscopist recognizes cuticularized membranes by their insolubility in con- centrated sulphuric acid. According to AMBEONN, fat-crystals may readily be detected in the cuticle (cuticula) by means of polarized light. FIG. 35. Transverse section of Ribes nigrum from a twig one year old. e, Epidermis; h, hair-cell; r, bark -parenchyma; K, product of the cork-cambium c; fc, corkcells; pd, chlorophyll-bearing cells; 6, bast-cells. (After Sachs.) When and where is cork-formation necessary? Harmonizing with the properties of cuticularized cells-walls, a corky protective tissue is required on the following plant-structures : at points where the cuticle and epidermis are ruptured because of the growth in thickness of the stem or root ; on delicate plant-structures which 58 COMPENDIUM OF GENERAL BOTANY. are habitutally or accidentally exposed ; on the leaf-scars ; on injured plant-tissues; on subterranean organs which must be pro- tected against excessive moisture (for example, potato-tubers, older roots and 1 rootlets ). In the chapter on Reproduction the cuticu- larization of the outer coverings of pollen-grains and spores will be discussed. This secondary corky change has a bearing on the ability to resist atmospheric changes for a shorter or longer time (resting period). In all these cases a protective tissue is required. Usually this We tissue has the power of continuous regeneration. find corky tissues in older roots, in subterranean stems, on leaf-scars, and, most common of all, as a covering of the cambium-ring of growing tree- stems. Each of these cases we must discuss more in detail. Scar-tissue ( Wundkork). The prick of a needle into a developing potato-tuber, or into the young stem of a woody plant, causes the death of the injured cells, and perhaps of a few others in their immediate vicinity. In nature such injuries may result from stings or bites of various animals. The uninjured cells surrounding the injured part at once proceed to divide parallel to the injured surface, that is, tangential to the centrally located injured cells. For example, an injury resulting from the puncture made by a needle will develop a cylindrical covering of suberized cells. This scar-tissue separates the injured (in other cases diseased) portion from the normal tissue, and at the same time prevents the evaporation of moisture from the injured surface. Falling of Leaves. Before the leaves begin to fall in the autumn a " " scission-layer is formed between the base of the petiole and A the stem. separation of the cells of this layer causes the leaves to fall off. In a large number of instances the formation within the scission-layer of a plate of ice which subsequently 2 melts, causes the profuse falling of leaves noticeable in the fall. The A scission-layer is, however, not the protective covering. pro- visional protection is formed by a mucilaginous substance known as callus, which closes the vessels ; or by the ' tyloses," that is, certain cellular protrusions which grow into the vascular system from the cells of its immediate surroundings. Drying forms a provisional protection for the parenchyma (perhaps in connection with a chem- 1 Also on the root-tubercles of Leguminosw and Cycas revoluta. TRANS. 2 MOHL, Botanische Zeitung, 1860, and Sachs, Vorlesungen. TISSUES AND SIMPLE ORGANS. 5& ical metamorphosis which is not well understood). The final pro- tection, however, is afforded by the formation of a layer of cork, which in some cases begins to develop some time before the falling of the leaf ; in other cases it begins later, and permanently supplants the provisional protection. The above-mentioned phenomena offered great difficulties to NAGELI, who in his theory of natural descent asserts that a stimulus We gives rise to an organ. ask : 1. What stimulus calls forth the formation of a scission-layer? 2. What stimulus gives rise to the beginnings of scar-tissue formation, even some time before a scar is present ? To return to our subject, I will state for the benefit of those who wish to enter more deeply into these relations that the vascular system of leaves (" leaf-trace") of many growing trees is abscised three times, or even oftener, in the course of the vegetative period ; first by the falling of the leaf, then again a little below the leaf-scar by the above-mentioned scar-tissue formation, and finally still deeper in the interior of the cambium by the growth in thick- ness of the stem (this occurs repeatedly among evergreen conifers). The necessity for the cork-tissue formation on stems growing in thickness has already been indicated. In only a few instances can the growth of the cuticularized epidermis keep pace with the growth in thickness of the stem as a ; result it is ruptured. From this follows the necessity of a new, somewhat more deeply located, layer of cork to guard against excessive evaporation. The plant behaves,, if the expression may be allowed, as if it knew what would happen later. Such " knowing" is, however, excluded: the occurrence of suitable processes is only in obedience to natural laws given by the Creator. Human intelligence is capable of comprehending the teleological moment of these and similar adaptations. The causal- mechanics, the causa efficiens, of the development of cork-tissue is, however, unknown to us ; this is usually the case. Since the cambium-ring continues its activity for years, the cork-covering first formed shares the destiny of the epidermis ; it is ruptured, and again a substitute is formed in the interior : that is, other cells situated more and more toward the interior become suberized. One of the most useful exercises for the beginner in plant- anatomy is to find the exact location of the first cork-formation in stems and roots. Such investigations teach that in the stems the epidermis itself may give rise to cork-formation (ph in Fig. 36 COMPENDIUM OF GENERAL BOTANY. B = cork-cambium). Usual! j it begins in a more deeply located layer of the parenchyma (Fig. 85). In roots the seat of corkformation is, as a rule, found in the pericambium. Concerning this pericambium, we will at this point state only that it is a tissue one or more layers in thickness, lying within the primary root-parenchyma outside of the centrally located vascular bundle. Cork is a complex structure, com- posed of different elements, but its origin can be easily determined. As a rule, it is developed according to a twofold plan- either as ring-cork, or as scaly cork. From the nature of things tissues which are separated from the sap-bearing tissue FIG. 36.-Tw" stages of cork- of the interior by a corky layer are subject formation in the stem of Scu- to desiccation. It is also a rule that one and tellaria splendens. (After Haberiandt.) , ,. -/ the same cork-cambium does not possess an unlimited power of growth, as is the case in the cambium-ring of our trees. The cork-cambium discon- tinues its cell-forming activity, while a new zone of cork-cambium appears more in the interior ; this new layer bears the same relation to others, etc. Either these successive cork-layers have the form of continuous cylinders, in which case they appear as rings in cross-section, and the bark peels off in cylindrical pieces, or the successively formed cork-cambiums (and their products) have the form of watch-crystals or similar curved surfaces whose convexities are directed inward, appearing as partial circles in cross-section, and in some cases (Platanus, for example) forming scales whixjh peel off very perfectly, leaving the stem quite smooth; in other cases the scales remain attached in large numbers for some time, the bark becomes very rough with deep crevices, and the scales are thrown off at irregular intervals. Hence "bark" at first contains the elements of the primary parenchyma between its cork-lamellae, later also those of the secondary parenchyma, still later only those of the secondary parenchyma. Besides the above-mentioned conditions in the case of birch-bark (Betula\ thin-walled and thick- walled unsulerized cells, which are intercalated between the suberized cells, are sometimes formed in TISSUES AND SIMPLE ORGANS. 61 other plants. These are the so-called " " scissiou-phelloids of v. HOHNEL/ which have the function of bringing about the scaling of the bark. The scar-cork or scar-tissue has been mentioned above. When the living cells of various tissues are injured or killed, the neigh- boring cells are sometimes enabled to create a protective covering at once, having therefore the behavior of cork-cambium. It is not within the province of this book to enter into a discussion of prac- tical arboriculture. I cannot, however, omit pointing out the funda- mental principles underlying all those operations which are of such importance in fruit-tree culture, namely grafting. In the various kinds of grafting, such as root-grafting, side-grafting, saddle-graft- ing, bud-grafting, etc., injuries must of necessity occur ; while in all cases an effort is made to induce the separated parts to grow together. One essential to bring about such a union is that cambium must be in contact with cambium. The growing together of separated tissues sometimes takes place during the natural develop- ment of plants ; but caution is necessary in the explanation of such phenomena in order to avoid the mistake of pronouncing tissues as having grown together which were in reality never separated. The phanerogamic parasites form a growth-union with the host plants, while the basal parts of sympetalous (united petals) corollas have never been separate. Structural Aids to the Function of Cork-tissue and Cuticula (cuticle). Trichomatic Organs (trichomes). In harmony with the subject under discussion the question might arise, Are there still other structures, besides the epidermal system with its cuticular and cork-forrnations, which serve to protect plants against excessive drying ? As is to be expected, this question is answered in the affirmative. Among other works, the reader interested in this sub- ject will find valuable information in YOLKENS' " Flora of the Arabian Desert " (Berlin, 1887). I will touch briefly upon the salient details. The limitation of the entire life of desert plants to the most suitable period of the year (period of rainfall), therefore also the hastening of the vegetative period, then the transfer of the time of 1 Wiener akad. Sitzungsber., LXXVI, 1. Abtheilung. J. E. Weiss has also written on the same subject (Deukschrift. d. K. Bot. Ges. zu Regeusburg, 1890, VI). 62 COMPENDIUM OF GENERAL BOTANY. vegetation to the most suitable period of the year, will first be considered. The formation of roots reaching deep into the soil, the surrounding of the roots with sand and particles of earth by means of the root-hairs, which usually serve to take up food materials, the hygroscopic salts mentioned on page 56, the retention of rain and FIG. 37. A, Climbing hair-cell of Humulus. 7?, Climbing hair-cell of Phaseolus. C, Adjacent margins of two pappus-scales of Galinsfga parviflora. Ca, hair-cell of Urtica urens ; Cb, upper end of the same ; Cc, the same with tip removed at z. D, Scaly compound hair-cell from the leaf of Hippophae rhamnoides. E, Twining hair-cell of the calyptra of Polytrichum juniperinum. (After Haberlandt.) dew by means of the trichomes, must all be considered as means to the end under consideration. Along with these structural arrange- ments especially the arrangement for the taking up of water there are also adjustments for retarding the loss of moisture, such as the reduction of the evaporating surfaces the ; leaf-formation may be TISSUES AND SIMPLE ORGANS. 63 absent or reduced to a minimum, in which case the stem-parenchyrna alone carries on the process of assimilation ; other means are the rolling up, curling, or folding of the leaf-surfaces, the vertical position of leaf-blades, and the formation of mucilaginous substances in the epidermis for the purpose of retaining moisture. Later, in the discussion of the aerating system, we will learn to know another characteristic phenomenon occurring in various forms which has to do with the position and structure of stomata (the openings of the aerating system). This phenomenon also belongs to the above- enumerated arrangements for reducing the loss of moisture. The mention of trichomes made above lead me to make the following statement. The anatomy of trichomatic organs has been accurately studied ; their physiological significance is, however, not correspondingly well known. For that reason I shall conclude this chapter rather hurriedly. Of the great variety of forms of trichomes I shall select only a few represented in Fig. 37. If a glandular hair secretes an ethereal oil, its function seems clear, namely, to attract insects which will carry the pollen. If the secretion is of a sticky consistency it evidently serves to keep off injurious crawling insects, since these take the honey without aiding in cross-fertilization. The flattened or shield like trichomes which cover the breath- ing-pores evidently serve to guard against excessive loss of moisture. The satinlike shimmer of floral leaves is due to papillose trichomes (conical projecting epidermal cells). In some instances it has been proven that trichomes with thin- walled areas near the base serve to admit moisture (rain, dew). Still a considerable number of trichornestructures remain whose physiological significance is not satisfac- torily explained. III. FUNCTION OF MECHANICAL TISSUES. Even a superficial consideration of the plant kingdom suffices to teach that the mechanical influences surrounding land-plants, waterplants, aerial organs, subterranean organs, etc., are different, and that these various plants and plant-organs require definite adaptations as to the firmness of the tissues concerned in order that they (as the normal course of things teaches) may be maintained in their entirety. In upright stems in fact, in all organs which must maintain themselves in an upright or in a free horizontal position bending enters into consideration, especially as the result of air currents; also 64 COMPENDIUM OF GENERAL BOTANY. of the weight of the leaves and stems, of the snow, ice, etc. The roots of a tree through whose crown the wind blows, and the grass- stem the panicle of which offers resistance to air currents, are subject to & pulling tension. The margins of flat leaves waving in the wind are subject to tearing and breaking. Parts of winding stems wound about dead supports, and more especially those wound about living supports (tree-stems growing in thickness), and tendrils must resist pulling tensions; likewise water-plants in rapidly flow- ing water, and stems of hanging fruit. Rarely there comes into play a supporting tension similar to that of a pillar, as in the case of 1 supporting roots. The question now is, How are such mechanical requirements to be interpreted ? One difficulty will be to explain these interesting relations briefly, yet not at the expense of clearness. In many respects the brief suggestions given in these lines, in other cases a hasty outlining, will assist in finding the necessary explanations. While I shall attempt to demonstrate the mechanical principles in the internal structure of plants by giving a few examples, I shall base my discussion of the subject upon SCHWENDENER'S " Mechanical Principles, etc.," as well as upon NAGELI and SCHWENDENER'S " Microscope." As has been demonstrated (SCHWENDENER), there is in the vegetable kingdom a specific mechanical tissue-system, consisting of specific mechanical cells, which in its best quality has the same sup- porting power as malleable iron wire, namely, twenty kilos per square millimeter (within the limit of elasticity}. These mechanical cells are designated by different authors as : stereids, skeleton-cells, mechanical cells, thick-walled bast, hard bast, prosenchyma-fibres, 3 bast-cells, sclerenchyma-fibres. In organs subject to bending the mechanical cells are peripherally located, while in organs subject to a pulling tension they are centrally located ; that is, in typical cases they are arranged according to rational mechanical prin- ciples. That such an arrangement of mechanical cells is a rational one is made clear by the following elementary considerations (com- pare the accompanying figures, 38-42, as well as those pertaining to the root anatomy). 1 Aerial roots of Zea Mays afford a typical example. It does not seem clear why all vertical tissues are not subject to such a tension. TRANS. 2 No doubt we must wait some time before a uniform terminology will be adopted. TISSUES AND SIMPLE ORGANS. 65 T. The fibres and tissue-layers of a beam supported at both ends having a weight in the middle are so influenced that the uppermost fibres are most strongly pressed together and the lowermost fibres are pulled. In the middle of the beam in cross-section there is an imaginary " neutral" fibre in which the pressing tension passes into a pulling tension. In this region pushing and pulling are at a minimum. From this it follows that in order to have an appropriate distribution of material in such a beam it must, in general, have the form (in cross-section) of two capital T's, one of which is inverted, thus (S3), since the mass of material must be distributed at the points of greatest tension. In following out this idea one can readily understand that a hollow cylinder would represent a type of structure adapted to resist a bending tension from all sides. The combination of many double-T supports will give us a polygon whose sides are represented by the cross-lines of the T's. These ' cross-lines, as already stated, indicate the strongest parts of the sup- port (" girth ") ; the radial connecting lines (" filling ") may be much weaker; when the " girth" becomes continuous, the " filling" may be entirely omitted. II. In the determination of the equilibrium of a prismatic staff bent to one side by some lateral force we must first of all find the "modulus of elasticity." This maybe found as follows (it must be remembered that in the rational construction of this formula no fibre is to be stretched or elongated beyond the limit of elasticity) : A If we let W tested, represent the area, in cross-section, of the tissue to be the maximun weight which can be supported without permanent elongation, then the supporting power within the limit U= W U of elasticity per unit of surface -j. By dividing by the specific elongation due to W, that is, y, in which A equals the elonga- tion due to the tension and I the original length, the modulus of E = elasticity is found U. 1 -i-. A III. Besides the modulus of elasticity, there is still another factor which enters into the determination of the equilibrium of a bent twig or staff. 1 In normal well-developed bast 1000 units (in length) of I equal about 13 units U E= of A, = 20, hence 1540. 66 COMPENDIUM OF GENERAL BOTANY. FIG. 39. Cross-section of the stem of Scirpus ccespitosiis. (After Haberlandt.) B FIG. 38. Mechanical cells in cross (B) and longitudinal section (A). (After Haberlandt.) FIG. 40. Bast-ring of the stem of Molinia ccerulea. (After Haberlandt.) FIG. 41. Mid-rib of the leaf of Zea (After Haberlandt.) FIG. 42. Transverse section of the rhizome of Carex glauca. (After Haberlandt.) TISSUES AND SIMPLE ORGANS. 67 Let us make a simple experiment with a ruler. With one of its flat sides turned upward it may readily be broken by a force acting downward or upward. If the same force acting in the same direction acts upon the ruler with one of its edges turned upward, then it will scarcely be perceptibly bent. In the latter case forces in- herent in the woody fibres are brought into play to counteract or equilibrate the bending force; in the former case this does not occur. From this it is evident that in order to determine the point E of equilibrium there is besides another magnitude, the so-called moment of flexibility (Biegungsmoment). The latter ( W) is dependent upon the form and area of the transverse section. In the case of the ruler it is evidently the form of the cross-section, which differs in the two positions. TFis found by multiplying the area of each element in cross-section by the square of its distance from the neutral point, and then adding the number of such products in the entire cross-section. 1 (The limit or amount of flexibil- W ity to be determined experimentally depends essentially upon and E.} From the above illustration with the ruler it follows that the pressing and pulling forces (of opposite elements) resulting from a lateral pressure upon a beam is inversely proportional to the distance of the girdings. The supporting power of the beam increases not only with the strength of the girdings, but also with their relative distance of separation ; that is, the stronger the girdings the farther they may be apart in order to give a maximum effect. This principle of the peripheral arrangement of firm elements in supporting organs, though simple, is most extensively embodied in multitudinous forms in the arrangement of mechanical plant- cells. As it is customary in technics when using two different materials for example, wood and iron to place the stronger material where the greatest support is to be maintained, that is, at the girding, while the weaker material is used as filling, so it is found that mechanical cells in the supporting tissues of plant-organs are peripherally arranged, while other tissues which serve the purpose of nutrition, storing of food material, conduction of fluids, etc., represent the filling material. It is to be expected that the assimilating system, dependent upon sunlight for its activity (chlorophyll-bear- A 1 Let equal the area in cross-section of one element, r its distance from the neutral point; then A. r* = the moment of flexibility of a given element. TRANS. 68 COMPENDIUM OF GENERAL BOTANY. ing tissues), and which is also peripherally located, must make suitable concessions to the mechanical tissues as to position. This is what actually takes place. IY. Theoretically the strength of a given support would depend only upon the magnitude of its cross-section. It is, however, evident that six silk threads which are about one cm. apart and so placed that one is central and the other five peripheral, are in danger of being torn by some pulling force, because tension on them is