Mechanical Tissue and Stem Flexibility

the cambial portion of the inter- node. At co the mechanical tissue- system is not composed of typical bast but of collenchyma, therefore capable of growth. If a grass-stem is placed in a horizontal position, the more rapid growth of the lower side of this collen- chymatous node will cause it to rise to a vertical position. The firm and more mature portion of the stem within co is thereby passively bent. A rare case, occurring known only in the genus so far as 1 Armeria, has been observed and may be briefly mentioned. The mechanical sheath of bracts at the base of the inflorescence extends from above downward the ; growing part of the peduncle lies at the FIG. 68. through Longitudinal section the node of a grass- upper end of the internode. The stem. sheath is completely formed in the (Diagramatic after Schwendener.) young plant, and after the peduncle has completed its growth it dries up and finally becomes torn. 1 Reported by the author in 1881 (Monatsber. der Berl. Akad.). Professor Schwendener, who has doue so much for scientific teleology, during on<? of his ex- cursions in the vicinity of Berlin, expressed an opinion, as to what was probably the true state of the case, which led me to make more exact investigations. 122 COMPENDIUM OF GENERAL BOTANY. A third means of protecting areas of intercalary growth may be mentioned, namely, the increase in diameter in the region of the growing zone. Tradescantia erecta, according to Sehwendener, is one of those plants in which basal growth of the internode takes place ; it has internodes in the form of truncate cones. Exact measurements in regard to the course of intercalary growth have not yet been made. With reference to the intercalary (basal) growing leaves, which include the great majority of leaves, it may be stated briefly that the growing areas are protected by the enveloping sheath-like leaf- blades (elongated monocotyledonous leaves) as well as the overlapping of the leaves in the bud (dicotyledonous leaves). Among conifers there are membranous sheaths consisting of the bud-scales which enclose the leaf-base. VI. FOOD-SUBSTANCES DEBITED FEOM THE ATMOSPHEEE. ASSIMILATION OF CABBON IN GEEEN OEGANS. The dry (solid) substance of the plant-body is, for the most part, the transformation product of atmospheric carbonic acid (CO 2 , carbon dioxide). Pure carbon (C) constitutes about one-half of this dry substance, and is found in chemical union in the cellulose of membranes, in starch-grains, in fats, in plasm, etc. C appears CO CO in the green organs as gaseous a. 2 unites with the elements of water (H2O) through the influence of sunlight on chlorophyll, forming starch or some allied compound and setting free oxygen (O). These transformations take place in a very short period of time. The greater part of the plant-body (plasm and cell-walls) is therefore derived from the atmosphere. That carbon dioxide and water form the starting-points for the production of starch as well as for other related substances, with liberation of O, is well known; also that these transformations may take place in a few hours or minutes. But the most discern- ing chemists at present refrain from attempting to explain the individual chemical reactions involved in the important processes of assimilation. In general, the following formula may be con- + = H O + O sidered as correct: 12CO 2 10II.O cC2 20 10 (starch) 24 , while the gas- volumes remain nearly the same. TISSUES AND SIMPLE ORGANS. 123 The most important bearers of the assimilating function are the chlorophyll-bodies, which have a discoid form, among algae a band- A like, flattened or stellate structure. definite tissue in which they may occur is not always necessary, though we usually speak of assim- ilating cells, forming a specific assimilating tissue which is found in the true assimilating organs, the green leaves. The principles which underlie and regulate the function of chlorophyll-bodies, that is, the conditions under which they can perform their most favorable activity, also underlie the structure of the tissues and the We organs which serve the function of assimilation. will there- fore next consider the structure of the assimilating tissue-system. (a) The Structural Principles of the Assimilating System. G. HABERLANDT and STAHL have within more recent times made important investigations in regard to the physiological anat- omy of assimilation ; to these, among others, must be added the communications of HEINRICHER. From & physiological standpoint the communications of SACHS are the most important. The Greatest Possible Utilization of the Luminous Effects of Sunlight by the Chlorophyll. Following in thought the problem suggested by this statement leads us to the essential points of view which give us a physiological understanding of the structure of the When assimilating organs. I say '' greatest '' possible utilization of sunlight, I wish to explain, in order to avoid erroneous concep- tions, that the nature of chlorophyll is such that assimilation reaches its optimum with certain light-intensities; beyond these an injurious CO influence makes itself felt. Similarly with the amount of 2 present : increasing it to 8 per cent with high intensity of light there is still noticeable an increase in assimilation. Physiology must here likewise be satisfied with a causal-final or teleological ex- planation of the anatomical adaptations ; a causal-mechanical ex- planation is impossible. The principle of the surface expansion of the leaf which mani- fests itself by the outer form is still more evident in its anatomi- A cal structure (Figs. 69, TO, 71). maximum expansion of cell- surface is obtained by membrane-foldings, by the regular form and large numbers of assimilating cells. Such adaptive arrangements make room for the numerous chlorophyll -grains which are always adjacent to the cell-wall. The regular, elongated palisade-cells. 124 COMPENDIUM OF GENERAL BOTANY. with their extensive cell-wall areas, the 4' '' arm-palisade with its foldings (or incomplete cell-walls), are also formed on the principle of great surface expansion. It is of course necessary that these cell-wall surfaces occur on the side of the leaf exposed to sunlight. The arrangement of these walls and their foldings are also to be considered in their relation to other requirements ; first of all they serve to conduct the products of assimilation by the shortest route possible, and at the same time permit light to pass to the more deeply seated cell-layers. There is, no doubt, a reciprocal relationship between the light-intensity and the perfection of the assimila- tory tissue-system, in that the constant lateral position of the chlorophyll-bodies in the palisade-cells (movement of the chlorophyll-grains within the palisade-cells is only an exceptional phenomenon) is most suitable for strong light-intensities (Stahl). However, the structural conformation to strong light-intensities does not take a higher rank than that for conveying food-substances by the shortest route possible (Haberlandt). That the latter is indeed a principle of prime importance can be seen by glancing at the figure of Silphium laciniatum (72) ; further, also, from the fact that there is a group of plants in which the assimilating cells are at the same time conducting cells ; they extend parallel to the leaf- surface, either in a direction toward the leaf-base or toward the median vein (leafy mosses, some monocotyledons). In the case of Silphium (see Fig. 72) we can see that the posi- tions of the cell-wall bounding the intercellular spaces (2), although eventually exposed to strong illumination, are lined with chlorophyll-grains, while the portions of the cell-wall which cross the current of assimilates at right angles are free from them : this is an example of the predominance of the principle of conduction. Finally, there are cases in which the palisade- cells are radiately arranged about a vascular bundle, which unmistakably indicates that the principle of conduction by the shortest route possible is of prime importance. The palisade-cell placed at right angles to the leaf-surface is only a very frequent special case in the series of elongated assimilating cells. With reference to these adaptive relations we shall, with HABERLANDT, place the arrangement and position of the palisadecells under the principle of conduction by the shortest path. STAHL is inclined to consider the adaptation to light-intensities as the most TISSUES AND SIMPLE ORGANS. 125 FIG. 69 A. Vertical section the leaf of Sambucus " nigra. th"rAorumgh- palisades. (After Haberlandt.) FIG. 71. Vertical section through the leaf of Juglans regia. Palisade-cells are supposed to be richly, the spongy tissue cells less richly, supplied with chlorophyll. Both cell-forms are here typically developed. (After Haberlandt.) FIG. 69 B. Vertical section through the leaf, including the midrib, of Raphanus sativus. (After Haberlandt.) ]?IG 70. Vertical section through the leaf of Ficus elastica. The epidermis is omitted. v and pi, Palisade-cells ; a, collecting cells ; s. p sheath ; y, vascular bundle. (After Haberlandt.; FIG. 72. Lower surface of an *' isohiternl " leaf, Sil- phi'iim laciniatum. (After Haberlandt.) 126 COMPENDIUM OF GENERAL BOTANY. important. Nor do we deny the correlation of the adaptations. The arrangements of the palisade-cells at right angles to the leafsurface is the most common position of assimilating cells, because here illumination is as a rule most perfect or intense ; furthermore, the adaptive development of one side favorable to light in leaves illumined on one side, and the adaptive development of two sides favorable to light in leaves illumined on both sides (isolateral leaves), are additional evidence of this correlation, and, in general, the cor- relative arrangement of typical assimilating cells. Finally, light-in- tensity and anatomical structure give expression to this correlation in the differences of sun-leaves and shade-leaves which develop on the same plant or plant species. The difference is particularly noticeable in the stronger development of the palisade-tissue in the sun-leaf (Stahl). In addition to the two structural principles of HABEKLANDT surface expansion and shortest path for the assimilates we may add a third structural principle harmonizing with existing facts, namely, Stahl 's principle of the adaptation to light-intensity. From the above arguments we must consider this relation to light as a struc- tural principle belonging to this chapter. In one respect these three principles are very much alike : all are readily understood from a teleological standpoint, not one is explained casual mechanically. The factor light must invariably be brought into consideration. Below the palisade-cells of the luminous side of an ordinary horizontal leaf lies the loose spongy tissue, so named, because of the large intercellular spaces and irregular cell-forms. This structure, which is also shown in the accompanying figures, is characteristic of the lower surface of the leaf. It evidently serves to perform several functions : (a) the conveying of the products of assimilation to the parenchyma-sheaths of the vascular bundles ; when the proc- esses of differentiation have progressed somewhat more, we may also distinguish " collecting cells " (see Fig. 70, a); (5) an assimi- We lating activity because of the chlorophyll present. must also bear in mind the self-evident result of the bounding of numerous cells by intercellular air-spaces, that is, considerable transpiration must take place. The author, however, agrees with YOLKENS who looks upon this transpiration as a process physically necessary and which produces physiological effects, but which in itself is not We a physiological function. must, however, ascribe to the TISSUES AND SIMPLE ORGANS. 127 spongy tissue a third essential function, namely, the aeration of the typical assimilating tissue (the palisades). The latter contains numerous but narrow air-channels which are arranged about each CO palisade- cell ; but the supply of and 2 the nearest centres of O accumulation for the liberated are naturally to be sought for in the spongy tissue, since it communicates directly with the atmos- phere. Particulars will be given below (VII., Special Function). The teleological consideration of nature suggests that not all leaves met with in nature are built ' ' bifacially ' ' and equipped with anatomically different light- and shade-surfaces. Observation teaches that beside the large number of flat leaves placed horizon- tally there are many of cylindrical form (linear leaves), and others which are flat, but not horizontal, either having the margin turned toward the stem (CalUstemon, Lactuca scariola) or placed ap- proximately vertical. The latter position occurs among some grasses, among orchids, in Acorns, etc. From this may be deduced 1 the following : A A. "centric" type of structure with a two-sided or cylin- drical evenly developed chlorophyll-bearing parenchyma is peculiar to those flat leaves not horizontally placed, as many grasses, or- chids, Acorus, Lactuca scariola, Callistemon, etc. (see Fig. 72). Also those leaves approximately cylindrical needles, so called. To the latter should also be added the green culm-like stems (hal- martige /Stengel). B. The majority of leaves belong to the bifacial type and are always flat and placed horizontally. I shall not hesitate in citing a very striking example of adaptive phenomenon. The leaves of Allium ursinum, Alstrwmeria, and others, in their early develop- ment cause the morphologically lower surface of the leaf to be turned upward by a torsion of 180 of the petiole or leaf-basis. In these leaves the morphologically lower surface possesses the An structural arrangements for active assimilation. analogous example has been observed by SCHWENDENER in the mechanical adaptation of the leaf of Gynerium argenteum. The same physiological significance as that of the mormal bifa- cial leaf-structure also underlies the fact that in the lichens the assimilating algal cells ("gonidia") are found nearest the luminous side of the leaf-like thallus (see the chapter on symbiosis). 1 DE BARY, Comparative Anatomy, page 406, et seq. 128 COMPENDIUM OF GENERAL BOTANY. The physiology of the phenomena of movements will acquaint us with adaptive movements which will bring the leaves into the most suitable positions with reference to the sunlight. The chlorophyll-bodies themselves have special adaptations for the maximum utilization of the sun's rays. (5) Movements and Changes in Form of Chlorophyll-bodies. According to STAHL, the chlorophyll-bodies among certain forms of the so-called lower plants (filamentous algse) are capable of movement. In Mesocarpus each cell possesses a rotating chlorophyllplate which bisects the cell longitudinally. In diffuse sunlight the flat surface is turned toward the light, while toward the rays of direct insolation a profile position is assumed. In the single- layered leaves of the moss Funaria hygrometrica the chlorophyllbodies assume a position along the lateral walls (profile exposure) in direct sunlight as well as in the dark, while in the ordinary diffuse sunlight they are adjacent to the outer walls (surface exposure). In the palisade-cells of the higher plants it has been observed (mainly according to STAHL) that the approximately hemispherical chlorophyll-bodies with their flattened surfaces directed toward the cell-wall (longitudinal) extend, that is, elongate, somewhat more into the interior of the cell in diffuse sunlight, while in direct sun- light they lie more closely in contact with the cell -wall and increase their diameter in the direction of the adhering surface. Covering a leaf-portion with tinfoil causes this part to become more darkgreen as compared with the strongly illumined portions (SACHS). The following deductions may be drawn from the three phenomena illustrated by the above examples, namely, the rotating of the chlorophyll-plate, and the movements and change in form of the chlorophyll-bodies: 1. Chlorophyll is enabled to derive a maximum benefit from definite light-intensities by enlarging its surface area. 2. It protects itself against light-rays of too great intensity, very probably because it would thereby be injured in its function and composition. According to PKINGSHEIM, chlorophyll is destroyed by concentrated sunlight in the presence of oxygen. (c) The Chemistry and Physiology of Chlorophyll. The exact chemical composition of the green coloring substances designated as chlorophyll is but little understood. It contains the TISSUES AND SIMPLE ORGANS. 129 N, elements (7, //, 6>, and iron is necessary to its development (as well as to its composition?). The plasmatic colorless or nearly colorless basal substance (stroma) of the chlorophyll-body is tinged with the green coloring substance ; this latter can be extracted with alcohol. The delicate structure of this fundamental substance according to more recent authors is said to be spongy, not homogeneous. The fact that chlorophyll-bodies divide has been known for some time. Further, it has been supposed by many authors that two coloring substances, a green and a yellow, are present in the chlorophyll-bodies (according to earlier investigators, blue and yellow). The foregoing statements represent, so to speak, successive stages, which are not yet concluded, of the attempts made to find the. chemical and physical structure of chlorophyll-bodies. It is to be kept in mind at present that chlorophyll is a green-colored plasm of highly characteristic properties which manifest themselves in the work of assimilation. In regard to this work of assimilation we must, in view of the results obtained by ENGELMANN (Utrecht), admit that con- siderable progress has been made. The theory of the physicist LOMMEL that the rays which are absorbed by the chlorophyll-spec- trum are most active in assimilation seems to have been verified by Engelmann. The method of investigation of this latter physiolo- gist is in itself very interesting. It is called the '' bacteria '' method, and consists in its essentials of the utilization of sensitive bacteria suspended in a drop of water. The bacteria accumulate where An there is a supply of oxygen. assimilating cell-thread under the microscope is observed under such environments as expose it to the seven colors of the solar spectrum which are projected side by side on the long axis of the thread ; the surrounding liquid con- tains the sensitive bacteria ; they accumulate most at the points of maximum assimilation, hence where the most oxygen is liberated. These experiments show that the two optima of assimilation (as judged by the liberation of oxygen) occur first in the red and a second smaller optimum occurs in the highly refrangible parts of the spectrum : blue, violet, and ultra-violet ; it is in these spectral areas that the characteristic absorption-bands of chlorophyll lie (similar to those of living chlorophyll). (The optimum of assimilation in the red [orange] had been observed by BEINKE, previous to the investigations of Engelmann, and still earlier by ~N. J. C. 130 COMPENDIUM OF GENERAL BOTANY MULLER). All observers agree that assimilation is much less active in the more strongly refrangible half of the spectrum uni- formly designated as ' ' chemical '' rays (actinic rays) because they induce certain chemical processes than in the less refrangible half. The above coincidence of light-absorption and assimilation in the chlorophyll-bodies harmonizes with the l supposition that (1) there are certain atomic groups in the chlorophyll which are set in strong vibrations by the red, and less strongly by the more refrangible, rays of the spectrum, and (2) it is these atomic groups which do the work of assimilation fiy the transformation of light- waves into chemical activity. In connection with (1) we might mention the phenom- enon that an alcoholic solution of chlorophyll fluoresces with a red light, while the living green plant s does not fluoresce ; that is, it does not emit a red light, because the necessary vibrations are being transformed into chemical activity. The coloring substance chlorophyll and living plasm work together in the processes of assimilation: chlorophyll acts perhaps after the manner of a ferment. The history of assimilation also contains the investigations of PRINGSHEIM 2 which created considerable interest at the time. Pringsheim' s hypothesis has, according to my knowledge, no firm adherents. The peculiarity of this hypothesis is the original con- ception that the coloring matter of chlorophyll is only of physical importance, not chemical, and that it is the colorless plasm which is active in assimilation. According to Pringsheim, chlorophyll regulates the respiration of oxygen in plants by the absorption of the so-called "chemical" rays (blue, violet, ultra-violet), so that the activity of such respiration is reduced below the activity of assimilation. The absorption-bands in the red therefore cannot have the significance mentioned above. The optimum of assimilation, according to Pringsheim ; in agreement with SACHS and PFEFFER, does not lie in the red spectrum but in the yellow. In this matter we are far from having uniformity of opinion. But we will for the time being adhere to the opinion expressed above, which is based upon the results of Engelmann's and Reinke's experiments. 1 See HOPPE-SEYLER, Botanische Zeitung (1879), p. 819. 2 Sitzungsberichte der Berliner Akademie, 1879. TISSUES AND SIMPLE ORGANS. 131 We shall now further discuss the process of assimilation. Each individual chlorophyll-grain may be designated as a work- shop of assimilation. The chief requirements for this assimilation CO in the chlorophyll are the presence of 2 and the influence of diffuse or direct sunlight. Water is already present in the assimi-^ lating cells. Starch ' (amyliim) in the form of starch-grains is, in the majority of instances, the rapidly formed product of this assimilation, though it is not the immediate chemical product. Before solid starch-grains can be formed there must be a product of assimilation, also a carbohydrate, which is soluble in water, as some form of sugar ; even this may not be the first chemical prod- uct. The experimental-physiological fact that there is a volume CO of oxygen liberated approximately equal to that of 2 taken in, is in harmony with the assumption that a carbohydrate is the + = + H O O product of assimilation : 1 2CO 2 10H 2 24-O C W 10 10 . According to recent investigations (ARTHUR MEYER), the formation of soluble carbohydrates (devoid of starch) predominates in the chlorophyll of monocotyledons, while starch-formation predomi- nates among dicotyledons. In regard to the immediate, still un- known, product of assimilation we may state that, according to the H CO hypothesis of BAYER, O and 2 2 first unite to form an aldehyde (alcohol), and this is polymerized into a carbohydrate (CO2 -f- H = + CH H O O 2 2 O 2 O). LOEW produced a sugar (C 6 12 out of 6) the aldehyde formed from formic acid and limewater. U nder favorable circumstances starch-formation may take place in a few minutes. The starch that is formed will disappear in CO the dark, also in the light in the absence of 2 . Among many plants the starch formed during the day is carried into the petiole of the leaf and other tissues during the 2 night. Chlorophyll-grains as the workshop of our most essential food- substance, bread, deserve special attention. Our present scientific knowledge does not enable us to furnish even an approximate substitute should the above-described chlorophyll activity cease altogether. Science does not even comprehend the chemical 1 As a note on microcbemistry may be added. Iodine is only slightly soluble in water, more so in solution of KI or alcohol. All these solutions, more particu- larly the stronger, serve to demonstrate the presence of starch both microscopically and rnacroscopically by a blue or dark-blue coloration of the starch-grains. 2 For particulars see the works of SACHS. 132 COMPENDIUM OF GENERAL BOTANY. methods by which we are so amply supplied with "daily bread." Much less is it capable of imitating the process artificially. Chlorophyll proves to be of great importance during various- periods of chemical activity in 1 plants. Among trees with decidu- ous leaves we see that the assimilating organs are destroyed at the close of the vegetative period. Chlorophyll itself is, however, not simply lost; in the autumn before the leaves begin to fall the most valuable mineral constituents (kalium, phosphoric acid) pass into the enduring portions of the plant, to be again utilized the following year ; yellow grains, causing the autumn coloration of so many 2 leaves, remain in the cells of the falling leaves as a waste product. Chlorophyll-grains therefore undergo decomposition. VII. THE FUNCTION OF AERATION. The discussion of the fact that gas-forming and gas-requiring processes take place within the cell, and the explanation of a few simple observations associated therewith, will enable us to under- stand correctly the structural arrangements to be discussed below.