Let us learn about Anatomy of the Embryo and Young Seedling in Flowering Plants. After reading this article you will learn about: 1. Anatomy of the Cotyledon 2. Anatomy of Mesocotyl 3. Anatomy of the Sheath of Cotyledon 4. Anatomy of the Hypocotyl 5. Anatomy of the Seedling Root.
Anatomy of the Cotyledon:
The traces of the cotyledon vary in number. Commonly only two traces are found in the lower taxa of both monocotyledons and dicotyledons. Four is a frequent number, with transition to three by the fusion of the middle pair. Three is also common with the strong central bundle sometimes double.
The large number of traces are probable uncommon (e.g., Canna and some Araceae). When four traces are present, the two median traces lie close together. The Iridaceae have a median trace, simple or double, with one or two pairs of laterals. The Araceae have one to many traces.
The Liliaceae have all stages in the origin of an odd number of traces—two traces, four traces, a double traces a median trace with one or more pairs of laterals. Odd numbers result from the fusion of two or more traces. Number, position and fusion of traces coincide with those of leaf traces.
A single trace is characteristic of highly specialized cotyledons. In many taxa, commonly, the lateral bundles are free distally and with the median bundle, formed a three veined cotyledon. The two free laterals become major bundles in the anatomy of many specialized cotyledons.
The characteristic feature of many monocotyledons is the presence of two strong lateral traces that supply the sides of the sheathing leaf base. The median vein that supplies the cotyledon tip bends downward to the scutellum.
The lateral veins continue upward in the sheathing wings, or rarely they also continue downward to the scutellum. The tip of the median bundle, remained un-branched in small and cylindrical forms, but becomes branched in large forms.
In most cotyledons which possess two or three free traces, all the vascular bundles continue as veins towards the tip. In the down-bent cotyledons where the tip has been transformed into a scutellum, all the bundles may continue in the scutellum, but usually only the median bundle extends to it, the lateral bundles are shorter and their distal parts re-curve below the point of down bending.
The down bent cotyledons which show the prominent sheath as a part of the cotyledon, the lateral bundles are also prominent.
An even number of traces in the cotyledons of angiosperms denotes the primitive condition. The primitive taxa (e.g., Ranales, Liliales, Helobiales) have cotyledons with two strong traces, often with pairs of lateral traces. In most gymnosperms even numbered traces are characteristic. In angiosperms the odd-numbered traces are derived from even-numbered traces.
The number of cotyledonary traces is fairly constant in families. For example, two traces are commonly found in most of the lower monocotyledonous families. The Liliaceae have two in the lower tribes; two, four and six in others. The Zingiberaceae have two traces. The Amaryllidaceae have two; the Iridaceae, one, two or three.
The Araceae have one to several and the Cannaceae have several. Among dicotyledons the woody Ranales have two traces. The vascular system of the cotyledon of monocots has two major bundles or two with additional vascular bundles. The cotyledons of dicot have commonly three major bundles.
Anatomy of Mesocotyl:
The mesocotyl is a compound structure which consists of the hypocotyl and the adnate part of the cotyledon. It contains the vascular tissues of the stele of the hypocotyl and one or more vascular bundles of the cotyledon. The external and internal effect of adnation of the cotyledon neck to the hypocotyl is frequently found.
External evidence of the fusion is the presence, in some genera, of a longitudinal ridge on the axis of embryo; internal evidence is the presence of a vascular bundle running longitudinal in the cortex of hypocotyl.
The superfluous bundle belongs to the down bent adnate neck of the cotyledon that makes the vascular supply of the scutellum, the median bundle of the cotyledon continued downward from the point of down-bending of the cotyledon neck.
The vascular supply of the scutellum thus follows a roundabout course, like an inverted V. With the result of the bending downward of the tip of the cotyledon, the adnate vascular bundles, as seen in cross sections of the mesocotyl are inverted the xylem external to the phloem.
When the down-running bundle of the cotyledon is merged with the vascular tissue of the hypocotyl, its free tip, which supplies the scutellum, is attached at the base of hypocotyl. In a monocotyledonous embryo (e.g., Zea mays) the scutellum represents the cotyledon; the mesocotyl represents the first internode of stem; the coleoptile represents the first leaf; and the hypocotyl represents a mere plate of tissue.
Anatomy of the Sheath of Cotyledon:
The term sheath of the cotyledon or cotyledonary sheath has been applied to the cap-like or sheath-like structure, the coleoptile, that encloses the plumule, and to the basal part of the cotyledon. The anamotical study of the cotyledonary sheath supports that the sheath is the basal part of the cotyledon.
Anatomy of the Hypocotyl:
Very occasionally the statement is given that the monocots have no hypocotyl. The basis for this statement is the shortness of the hypocotyl of many grasses that are commonly used to illustrate monocotyledonous embryos.
Very short plate like hypocotyls of monocots has received little attention anatomically, because of the complex structure where the transition occurs abruptly and the vascular strands lie almost horizontally.
The hypocotyl of some monocots has been described as ‘often plate like hardly existent’. Sometimes, in certain monocots it is long and prominent in the embryo, especially where it is a part of mesocotyl. The hypocotyls of dicotyledons are generally longer than those of the monocotyledons.
The cylinderical or plate-like hypocotyl is greatly distorded and it tends to be ovoid or spherical in seedlings of Crocus and Raphanus and in poorly differentiated embryos of orchids, saprophytes and parasites. The types of transition in vascular structure between root and stem—from radial and exarch to collateral and end-arch.
Anatomy of the Seedling Root:
The vascular cylinder of the primary root of seedlings is commonly diarch or tetrach. Monarchy is rarely found in primary roots. Polyarchy is frequently found in monocotyledons. It is believed that tetrarchy is basic type since it is associated with arbore-scent taxa.
The diarchy is associated with herbaceous taxa. The Ranunculaceae have diarch condition, whereas the woody families have tetrarch primary roots. In the dicots there are few variations from diarchy or tetrarchy. However, in the monocots there are many variations, but polyarchy is common, the neck, the median part, differentiated as the coleoptile, and the apex, the scutellum.
In cotyledons where there is no down-bending of the distal part, all the vascular bundles including those of the sides of the sheath, usually continue into the scutellum. These cotyledons, with two or four traces are simple and of primitive type. In cotyledons of advanced type, with one trace which divides to form a median and two lateral strong bundles, only the median bundle continues to the scutellum.
The branches extent first upward and laterally and then downward, on the opposite of the sheath. The course of the bundles has been considered remarkable and evidence that the coleoptile is an independent organ, not a part of the cotyledon. The position and course of the veins of sheath support the interpretation that the upper part of sheath is stipular.
Where there is adnation of the cotyledon neck laterally to its lower part, the midvein is bent back upon itself. The two parts are fused and a bicollateral bundle is formed. Under continued specialization of this vascular fusion, the bundle is shortered distally and may become united with vascular stele of the hypocotyl.
The shortening and adnation may extend to the point of origin of the branches of the trace of the sheath, and the branches then appear to arise directly from the hypocotyl. The origin of the trace of the sheath from the hypocotyl, shows that the sheath and the coleoptile represent the first leaf.