Le Basi Anatomiche Dei Rapporti Nutritivi Tra Pianta Madre,Seme Ed Embrione

Abstract

The anatomical basis of the nutritive relationships between mother plant, seed, and embryo. — The morphology and anatomy of the fruits and seeds of the Angiosperms show a great variety of structures and adaptments, even within the same family, and one must be cautious in drawing generalized conclusions.

If we first examine the ovary we see that the single carpel receives three vascular traces from which the three main bundles originate, a dorsal and two ventral ones, all more or less reduced. Except in the case of laminar placentation the ovule traces are connected to the ventral vascular system, but often the entire vascular system of the ovary is anastomosed and therefore reticular. However especially when the placentae are at the centre or at the basis of the ovary, it is possible to detect a tendency towards a separation between the vascular system of the ovarian wall and that of the placentae.

The ovular bundle runs through the funicle reaching the chalaza, where it can either end or continue towards the micropyle with a single bundle or with a few branches or even forming a complete reticular envelope surrounding the ovule. The ovular vascular bundles are normally found in the outer integument.

The ovule is made of an inner part (nucellus), and an outer one (integuments). The integuments play a very important role in the processes of seed maturation, dormancy, and germination. They are isolated from the interior of the seed by a cuticle which is a common production of the inner integumentary epidermis, and of the nucellus. The cuticle is not present in the chalaza and can be dissolved in the micropylar region: through these two apertures nutrients can penetrate into the seed or haustoria can grow out of it. During the course of maturation these openings become closed by various means, often through the formation of a new cuticle or of a suberised chalazal plate.

The nutrients which pass through the chalaza penetrate into the nucellus where in some cases one can find some structures which facilitate the communications between the chalaza and embryo sac. The endosperm feeds at the expense of the nucellus but often it can establish a direct contact with the chalaza or the integuments or even the placentae. This occurs often thanks to haustoria.

The embryo is normally surrounded at first by a more or less liquid endosperm: in a second stage the endosperm becomes cellular and the embryo grows at its expense through the digestive activity of the cotyledonar epidermis.

From an anatomo-physiological point of view the following points seem of particular interest:

(I) The endosperm and the embryo show a remarkable autonomy in respect of the mother plant: from an anatomical point of view this is shown by the isolation of the endosperm and embryo by means of a cuticular covering or substitutive structures and by the interposition of nutritive tissues between the vascular system of the mother plant and the endosperm.

(II) Given the importance of the inner cuticle its presence and its constitution should be ascertained in the various species having also in mind the properties of selective permeability shown by the testa.

(III) Two nutritive mechanisms exist: translocation of nutrients via the vascular system and the nutritive tissues, and digestion of surrounding cells. In the digestive phenomena it is important to explain the mechanisms by which only the right cells are digested and not the others.

(IV) The embryo very frequently is immersed at first in a more or less liquid endosperm and is later surrounded by a compact tissue; the nutritive mechanisms are probably different in the two cases.

(V) Two endospermic zones are often distinguishable: one having an haustorial or at least a digestive or elaborative function, and being typically non cellular; another zone, typically cellular, forms a tissue which is sooner or later absorbed by the embryo. The cellularization of this zone seems to coincide with the establishment of polarity and with the beginning of maximum growth of the embryo.

(VI) The relationships between the inner seed and the integuments is complex and there is a correlation between the histoanatomical and biochemical changes of these two parts during seed development. The modifications undergone by the integuments are important steps also towards the preparation of the seed to the processes of dispersal, dormancy, and germination.