THE BOTANICAL REVIEW VOL. XIV DECEMBER, 1948 No. 10
H ISTO-PHYS IOLOGICAL GRADIENTS AND PLANT ORGANOGENESIS
HENRI PRAT Botanical Institute, University o~ Montreal
CHAPTER I. GENERAL CONCEPT OF A SYSTEM OF GRADIENTS IN LIVING ORGANISMS
The Concept of Gradients in Physics; its First Applications to Biological Sciences
A gradient is the progressive variation of a factor in function of the position. For instance: along a metallic rod, heated at one extremity, a temperature gradient appears; when pure water is poured gently on a sugar solution, a gradient of sugar concentration is established in the mass.
Application of this concept to biological sciences was made by Boveri (14, 15) who distinguished a progressive chemical varia- tion in the cytoplasm of ,4scaris eggs. Child and his collaborators (20) expanded the idea by recognising a "gradient of metabolism" along the embryo axis. Many embryologists (20, 23, 37, 46, 81, 90) subsequently enlarged the idea, and the concept of an embryo- genic field was introduced. As Huxley (46) states : "Within these fields various processes concerned with morphogenesis appear to be quantitatively graded, so that the most suitable name for them is field-gradient system or simply gradient-fields". We shall examine here the application of these concepts to the study of plant organ- ogenesis.
In the body of a plant; gradients may be classified as follows (71) : a) Physico-ehemieal gradients, referring to physical or chemical
elements, such as temperature, osmotic pressure, pH, rH, water concentration, glucose concentration, etc.
b) Physiological gradients, concerning the functions of tissues, 003
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v/z., respiration, photosynthesis, growth rate, tropisms, etc. They indicate the changes of intensity in these functions, according to the location in the plant body.
c) Anatomical and histological gradients, which concern varia- tions in dimensions, nature and shape of cells and cell-groups. Some of them are quantitative, others qualitative. We shall re- serve the term "gradation" to designate complex variations which are the sum of many elementary gradients. For example, a chemical gradation may involve a series of gradients concerning the concentrations of carbohydrates, of oils, etc.
Expression o I Gradients in the Different Systems o I Coordinates
A stem produces roots at its base, leaves and flowers at its summit, thus displaying a polarity which involves an intricate set of axial gradients (11, 18, 42). Such modifications do not occur, however, only in a one-dimensional system. In a ribbon-shaped organ, for example, a grass leaf (67), there are definite variations in its structure and functions not only along its length but also in its width, the latter chiefly in relation to veins. Thus we must recog- nize a set of transverse gradients in addition to the axial ones. Together they constitute a two-dimensional field, governing all development and functions of the tissues. We may trace, in such a field, a network of isopotential lines (lines of equal properties) which at every point are perpendicular to the direction of the gradients concerned.
Furthermore, an organ possesses not only length and breadth; it also has thickness, which involves another group of gradients. We are thus obliged to recognize a tri-dimensional field, with series of isopotential surfaces, always perpendicular to the vectors ex- pressing the gradients in space.
If an organ is cylindric, as is generally so of stems and roots, it may be useful to employ, in place of the usual three axes, a system of polar co-ordinates. Thus, besides the axial gradients, a set of radial ones can be distinguished (Fig. 7, II ; 8, VIII, IX).
Interactions Between Gradients
After having separated the elementary gradients we must always bear in mind their re-synthesis, for all gradients are merely diversi- fied expressions of the progressive variation of the properties and of
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the functional equilibrium of living matter in the mass of cells constituting the plant. It is easy to conceive that they are all interdependent, directly or indirectly, the slightest modification in any one of them having unlimited possible repercussions on all the others.
The physiological gradients appear, to a certain extent, to govern the whole system. In the meristem of an embryo, the establishment of metabolic, mitotic and respiratory gradients is noticeable during the earliest stages of growth. These gradients give birth immedi- ately to the two other categories, via., physico-chemical gradients, resulting from localization of the products of their activity (oxygen concentration, rH, carbonic acid, pH, etc.) and anatomic-histological gradients resulting from their influence on the development of the young cells.
Reciprocally, these induced gradients react on those that gave rise to them, for the local concentration of any product, e.g., car- bonic acid or auxin, modifies all the functional gradients. Further- more, when a group of cells has differentiated in a given way, its specialized activity necessarily transforms the entire system of physiological gradients as well as the chemical ones. Such interac- tions may be represented thus:
Physico-chemical gradients r
caAPa'v.t~ ii. PI-IYSICO-CnEMICAL G~,DI~.Na'S Chemical Gradations
Pigment gradations. Some chemical gradations, revealed by variations in color, are readily seen. For instance, in Melampyr~m nemorosum (74) there is a progressive transformation along the stem, affecting both foliar morphology and coloration (Fig. 1). The stem base bears ordinary lanceolate leaves, definitely green, while the apex shows intensely violet floral bracts provided with acute lobes. Every grade of transition can be observed between these extremes.
This transformation first appears in two differentiated spots at the base of the lowest leaves of the flowering portion of the stem
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(Fig. 1, No. 3). These spots are noticeable by their translucency and by the indentations in the outline of the leaf provoked by their reduced growth. Upward along the stem, from one leaf to another, the differentiated regions occupy progressively greater and greater areas and become ever more deeply colored, first, in pink, then in violet. The last vestige of green persists at the extreme tip of the leaves, Nos. 10 to 15; in the uppermost leaves (from No. 16 up) all traces of green tissue have disappeared. With respect to growth, the length of the appendages decreases regularly from below upwards (Fig. 1, curve AB).
N~I 2 3 5 7 10 12 15 16 17- /6
l ,/ - . , .
/ ,,.~ P.tq,; , . ,~r A~il:twigs Aborted t'roits . . . . . blowing blossoms
flow~r~ flo, vers FZG. 1. Gradation of foliar morphology and coloration in Melempyrum
neraorosum (?4). The numbers refer to the leaves on the floriferous part of the stem. The leaves are figured without regard to their relative dimen- sions. Curve AB indicates their lengths in centimeters; hatched areas represent anthocyanin regions.
In Mela~npyrum we thus observe a conflict between two modes of growth and development of the tissues. The first concerns the vegetative form of the appendage and leads to the formation of a comparatively thick, opaque organ, rich in chlorophyll, lanceolated and entire. The second concerns the bracteal state and leads to a thin, translucent organ, poor in chlorophyll, producing abundant anthocyanins, tending towards a general pentagonal form with numerous acute lobes; it involves inhibited growth, chiefly in the longitudinal sense. This second form of evolution of foliar meris- tems operates under the influence of reproductive organs in the course of their development.
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The close association of these two foliar types and the gradual replacement of the first by the second are clearly the result of the fact that production of young leaves is a progressive and rather slow process, whereas the sexual crisis is a sudden, rapid phenomenon. When flowering occurs, the varous leaf-primordia are struck by this crisis at different stages of development. Those at the base of the stem have already reached almost their definitive shape and are therefore affected only in a minute portion of their body, i.e., in the undifferentiated cells persisting at the base of their blades. The leaf primordia at the apex of the stem, on the contrary, are still entirely in a meristematic, i.e., versatile stage when the floral crisis occurs. Thus every part of them is affected by the deep change of metabolism provoked by flowering, and their whole body develops into an appendage of the second type.
Other genera, e.g., Poinsettia, Nidularium, Amaranthus, exhibit similar colour gradients associated with morphological ones, thus clearly showing chemical gradations. They include many decora- tive species, remarkable for the splendour of their bracteal system.
Gradations o S essential oils. In some perfume plants, such as Rosa and Jasminum, the fragrant essence is localized in the flower, constituting a secondary sexual character just as do the pigments in the previous cases. In others, as in the Labiatae, it is elaborated by stems and leaves in specialized secretory hairs. But, even then, it is not indifferent to the sexual influence. For example, in La- vandula the fragrance is more delicate near the flowers. In orange tree the perfume is entirely different in the leaves, in the petals, in the fruit pericarp and in the vesiculous hairs of the endo- carp. It would thus be necessary to distinguish here not only one but a set of gradients concerning each of the odoriferous essences, all of them being under the influence of the reproductive process.
In some cases complex secretory gradients can be detected, even in minute groups of cells. For example, the glandular organs studied by Martens (57) in Polypodium virginianum secrete tannic products in their basal cells and resins and essences in their apical ones. In the epidermis of grasses such tiny chemical gradients, on a cellular scale, (67) will be pointed out later.
Detailed study of essential oil gradients would be of great value for the perfume industry, a difficult task considering the fugacious and unstable character of these products.
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Gradations o[ carbohydrates. Colin (21) and his students have carefully investigated the variations of carbohydrates in different plant organs. For example, in sugar beet, at the end of the day, the foliar parenchyma is rich in starch and sucrose. Toward the base of the big veins and in the petiole the sucrose concentration decreases, but, in compensation, glucose and levulose (fructose) increase steadily (Fig. 2, I). At the base of the petiole, sucrose
J 12 D
! I i
I lI FIG. 2. Gradients of carbohydrate concentration (21). I: sugar beet;
II : wheat. Solid curves indicate sucrose concentration ; dotted lines, levulose concentration. Arrows show the direction of migration. In II the leaf sheath is extroversed to make the diagram clearer.
practically disappears; but, a short space lower, in the tuberized root, its concentration abruptly rises up again, while glucose and levulose almost vanish. The transformation occurs in a ~urpris- ingly short distance, within a few layers of cells (Colin). Thus, once more, we observe the two kinds of gradients already seen in pigments and oils; i.e., slowly progressive or abruptly broken.
These gradients are modified acccording to the season and re- lated to growth phases. Leonard (49) notes that a high level of fructose corresponds to rapid growth. In potatoes there is great increase of starch concentration in the tuberized portion, and in
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carrots there are also significant carbohydrate gradients in correla- tion with bud and root development (62, 63). When buds are present, the carbohydrates of secondary tissues are translocated so as to produce new roots with well organized primary structure. When buds are sectioned, the tendency is toward an increase of old secondary tissues and formation of new secondary structures.
Not less interesting is the carbohydrate gradation along a stem during maturation of seeds. For example, in wheats (Fig. 2, II) the starch percentage increases toward the summit of the terminal internode and becomes maximum in the grain. In compensation the percentage of levulosans, which increases downward along the leaf-sheath, decreases during the ascent along the internode. Care- ful study of these gradients may lead to important agricultural appli- cations.
Gradations o] alkaloids. Chase (19) has observed notable vari- ation in the distribution of nicotine in the organs of tobacco plants. This alkaloid does not exist in the seed, except in the integument, but, as soon as germination begins, it appears. In young seedlings, three millimeters long, it is present in the growing points, in the hypocotyl and in the root, chiefly in the piliferous layer. In the stem cortex the alkaloid percentage decreases, first from the outside inwards, then increases again in the pith. On the leaves there is considerable exsudation of nicotine by hairs and stomata, chiefly around the ends of veins. We thus note in tobacco a definite set of chemical gradients, both axial and transverse, and their modifica- tions in function of the age of the tissues. They are correlated with ionic and oxido-reductive conditions of the cells.
In transverse sections of stems and roots, Priestley (76, 77) noticed the establishment of gradients of pH, the xylem being acid and the phloem relatively alkaline (Fig. 3, A). Between them there is a zone of intermediate pH, possessing the most favorable conditions for the conservation of meristematic activity. In this zone the protoplasmic proteins, their environment being near their isoelectric point, show a minimum tendency to swell and to combine with salts (89). This explains the position of undifferentiated, non-vacuolate cells of cambium between xylem and phloem and their peculiar mode of division. Their partition walls are likely perpen-
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FIG. 3. A: Disposition of ionic gradients in the transverse section of a young root (76). Solid black areas indicate acidic zones (xylem); dotted areas, alcaline zones (phloem). Arrows show directions of increasing pH (hydrionic gradient). Dotted lines indicate levels of intermediate pH where cambial activity is preserved. B: The same conception applied to a young stem. Sclerenchyma is figured, as xylem, in solid black. C: Oxidation gradients in the endodermis (85). Oxidase action, shown by stippling, is more intense on the outer tangential wall, and, in young stages only, on the Casparian strip. D: On a "gap cell" in front of a xylem pole oxidation is more intense and invades t...