Anatomia e desenvolvimento ontogenético de Coffea arabica L. var. typica Cramer

Abstract

O presente estudo anatômico de Coffea arabica L. var. typica Cramer tem por finalidade fornecer informações básicas necessárias ao estudo da anatomia comparada das principais espécies e variedades de cafeeiros, cultivadas no Estado de São Paulo. Nesta primeira contribuição o autor realiza o estudo anatômico detalhado dos órgãos vegetativos e reprodutivos da variedade typica, não se limitando apenas à anatomia descritiva dos diversos órgãos, mas também, sempre que possível, discutindo o desenvolvimento ontogenético das diversas partes do cafeeiro. No estudo da raíz e do caule procurou-se estabelecer a duração do desenvolvimento primário, assinalando o local de aparecimento, primeiramente do câmbio vascular e, posteriormente, do felógeno ou câmbio suberoso. Na discussão da anatonia das folhas mereceu especial atenção o estudo das domácias, sua morfologia e possível função. As flores são estudadas detalhadamente nos seus diversos elementos. Nos capítulos referentes à anatomia do fruto e da semente, além do estudo puramente descritivo das suas estruturas são ainda discutidas as diversas modificações verificadas durante o desenvolvimento do ovário e dos óvulos, respectivamente em fruto e sementes.

The knowledge of the anatomy of Coffea arabica L. var. typica Cramer should be considered as a basic contribution to the comparative study of the anatomy of coffee species and varieties cultivated in the State of São Paulo, Brazil. The detailed investigations carried out on the vegetative and reproductive organs of the coffee plant revealed the following. The roots at the end of the primary growth present a protostelic, poliarch (6, 7, 8, 9, 11 primary xylem bundles), and exarch structure, the following tissues or zone of tissues being visible: root cap, epidermis, exodermis, cortex, endodermis, pericycle, primary phloem and primary xylem. This primary growth has a very short duration and is very soon followed by the secondary growth, formed from a vascular cambium which makes its first appearance in a region localized approximately 5 cm from the root apex. Phellogen arises first in the pericycle and later in phloem and gives origin to a relatively thin periderm; this protective layer replaces the epidermis that is gradually killed and sloughs away. Lateral roots originate from pericyclic cells situated at the protoxylem outer edges. The origin of all stem tissues, as well as those of the leaves, can be traced back to a dome-shaped shoot apex, which measure 220-360 microns in diameter and 48-120 microns in its major height; this shoot apex interpreted under the terms of Schmidt's theory shows to possess a tunica composed of two or three cell layers, and a corpus, represented by a central core of irregularly arranged cells. Differentiation of meristematic tissue initiates at the distance of 800-900 microns from the shoot apex, but the complete primary tissue differentiation is only achieved in a region situated 2.5 cm from the shoot apex. Stem cross sections at this level exhibit the following tissue zones: epidermis, angular collenchyma, cortical parenchyma, pericycle, primary phloem, primary xylem and pith. No distinct endodermis is visible. The vascular cambium, firstly visible in a region distant 2.5 cm from the shoot apex, originates from procambial cells localized between the xylem and phloem, being at the beginning only fascicular; the cells of the primary rays undergo cambiform meristematic activity, and interfascicular cambium is thereby formed between the vascular bundles. Following a centripetal differentiation the cambial derivatives give rise to a continuous band of secondary xylem and secondary phloem. Older stem cross sections present a periderm which is about 70 microns thick. Cork cambium arises from the innermost cells of cortex and produces cork externally and phelloderm internally. Petiole presents a tricyclic structure, exhibiting three vascular bundles, the major one corresponding to the mid-rib; all the bundles show a normal orientation of the xylem and phloem tissues. Secondary vascular bundles are also present towards the wings of the petiole cross section. The structure of foliage blade is that of typical leaves of dorsiventral symmetry. At an early stage the blade consists of seven layers of relatively undifferentiated cells. At maturity, just one layer of palisade parenchyma and ten to twelve layers of spongy cells are present. Upper epidermis is uniseriate and formed only by epidermal cells proper; uniseriate lower epidermis exhibits these cells and stomata which are of the rubiaceous type, being formed by two guard cells and two subsidiary cells. Remarkable is the occurrence on the lower epidermis of certain structures called domatiae; the constancy of these structures on the lower epidermis of the leaves of Coffea and a few other rubiaceous genera has been considered an important taxonomic characteristic for genera identification. On the other hand, the domatia localization, shape, size and constitution, as well as the presence or absence of stomata on their outermost cell layer or the presence or absence of hairs on the borders of the domatia aperture, have shown to be important characteristics to be utilized in the differentiation of Coffea species and varieties. In Coffea arabica L. var. typica Cramer the ,domatiae are localized at the very angle formed by the secondary veins with the mid-rib, have a vesicular shape and open externally through an elliptical or round pore which measures 200-260 microns in diameter; the borders of the pore are nearly glabrous, a few hairs appearing only in a region distant from the borders; very few stomata are also present on the outermost cell layer of the domatia. Stipule structure is similar to that of foliage blade, but no differentiation between palisade parenchyma and spongy cells is apparent. The inflorescence is of the homotatic type, the lateral axes being very short; consequently the flowers are disposed in axilar glomerules; bracteoles are in number of two pairs for each flower set, being respectively lanceolate and triangular, and presenting the same structure of the stipules. Calyx is very rudimentary; the sepals resemble the leaves anatomically, and present only a trace supply; the petals are united in a tube forming a salver-shaped corolla, presenting a single vascular trace each. Stamens are epipetalous and have also a single trace as vascular supply; the anthers are two-celled, opening lengthwise. At maturity each anther exhibits in cross section four pollen-sacs, surrounded by an uniseriate epidermis, two fiber layers and a narrow strip of tapetum vestiges. Microsporogenesis is reported to be normal by several authors. Pollen grains are numerous, globose, with a very thick, smooth, and 3-4-sulcate exine. Pistil is represented by an inferior ovary, terminal style and two stigmatic branches. Ovary is normally two-celled, each one bearing a single anatropous ovule on a central placenta; funicle is very short, and the embryo-sac is of the normal type, monosporic, 8-nucleate. Anatomically the ovary shows to be formed by uniseriate epidermes, the outer one presenting stomata; between the epidermes there is parenchymatous tissue, and distributed in this thick parenchyma there are 2 or 3 concentric vascular bundle series. The coffee fruit is a drupe, containing normally two seeds; exceptionally there is the occurrence of three or more seeds in the cases of tri-celled or pluri--celled ovaries, or through false polyembryony when bi-celled ovaries present more than one ovule in each cell. On the other hand, by abortion of an ovule one-seeded fruit may be formed (peaberry). Ripe fruits have a fleshy and thick pericarp. Fruit development brings about a series of chemical and morphological modifications that lead to a reduction in the thickness of pericarp, from 1.5 mm in ripe fruits to 0.4 in dried fruits. Exocarp is represented by a single layer of hardened and lignified cells, with scattered stomata. Mesocarp is formed by several layers of polyhedric, large and lignified cells, the innermost of which are somewhat compressed and flattened. Amidst these cells are visible the vascular bundles showing a great amount of fibers. Endocarp is about 100 microns thick and constitutes in the ripe fruits the so-called "seed parchment". Studied in cross section the endocarp shows to be formed by 5-6 layers of intercrossing strong fibers, what gives this zone of tissue an extraordinary strength. Maceration allows the detailed examination of the individual fibers which measure 350-370 microns in length by 20-45 microns in transverse diameter. The cell walls are very thick and provided with ramiform pits; cell lumen is almost occluded. The coffee seeds or coffee beans are elliptical or egg-shaped, plane-convex, possessing a longitudinal furrow on the plane surface. Seed coat is represented by the so-called "silver skin", which ontogenetically originates from the primine, the single ovule integument. This seed coat is about 70 microns thick and anatomically shows to be constituted by an outer layer of fibers somewhat similar to the endocarp fibers; they are, however, shorter, measuring the longest ones 180-320 microns in length and 18-30 microns in transverse diameter. Their thick walls are provided with round, elliptical, or elongated simple pits. The size of these fibers, the thickness and number of pits on their walls are considered by Chevalier as important taxonomic characteristics for the differentiation of Coffea species. Completing the silver skin structure there are several layers of amorphic parenchyma tissue, and a layer of indistinct cells which originate from the innermost cell layer of the primine. On the other hand, the fibers had their origin from the outermost cell layer of the primine. Endosperm is foimed by polyhedric cells of very thick cellulosic walls, functioning the cellulose in this case as food storage. With the aid of a special technique plasmodesmata can be detected in the primary-pit-fields of the endosperm cell walls. The endosperm tissue seems to present differences in the structure and chemical composition of its various layers; thus, at the level of the embryo cavity the cells are flattened and elongated constituting a region which probably désintégrâtes during embryo development. The outer layers represented by hard endosperm slough away as a cap that involves the cotyledons of seedlings obtained from completely naked seeds. This cap resembles the seed parchment in gross morphology. The inner layers are considered soft endosperm. As to the chemical composition, the endosperm cells besides Water, contains protein, the alkaloids caféine and coffearine, oil, sugar, dextrins, pentosans, cellulose, caffetannic acids, minerals, various acids and minor constituents. The small embryo, localized at the bottom of the seed, on its convex surface is represented by an hypocotyl and two adherent cordiform cotyledons. Very seldom there is the occurrence of embryo with 3 or 4 cotyledons.