Endosperm - definition, types

 

                                                                        ENDOSPERM

Definition:

Endosperm is unique among the plant tissues with regard to its origin and development.

Endosperm is the most common nutritive tissue for the developing embryo in angiosperms.

Endosperm is the product of fertilization and is triploid. After double fertilization the egg is called zygote, and the fusion product of polars and the second ale gamete is termed as the primary endosperm nucleus. The former develops into an originated embryo whereas the latter gives to an almost formless tissue, the endosperm.

Types of Endosperms:

1. Nuclear endosperm
2. Cellular endosperm
3. Helobial endosperm
4. Cereal endosperm
5. Ruminate endosperm
   
   
   

Nuclear endosperm:

                In this type of endosperm the division of the primary endosperm nucleus and few subsequent nuclear divisions are not accompanied by wall formation. This results in a condition where the central cell of the embryo sac has formed a few to several thousand nuclei freely suspended in its sap. Such a condition of endosperm may persist toward the center. The degree of cellularization varies a great deal. Mostly the endosperm becomes completely cellular but in Phaseolus cellularization occurs only around the embryo. In crotalaria the wall formation is confined to the upper region of the embryo sac; the Chalazal region remains free-nuclear, and it often elongates and behave like an haustorium.

                Development of endosperm in coconut deserves special mention. The primary endosperm nucleus undergoes a number of free nuclear divisions. When the fruit is about 50mm long the embryo sac gets filled with a clear fluid in which float numerous nuclei of various sizes. At a later stage the suspension shows, in addition to free nuclei, several cells each enclosing a variable number of nuclei. Gradually these cells and free nuclei start settling at the periphery of the cavity, and layers of cellular endosperm start appearing. This forms the coconut meat. The quality of the cellular endosperm increases further by the divisions of the cells. In mature coconuts the liquid endosperm becomes milky, and it does not contain free nuclei or cells.

Cellular endosperm:

                The cellular endosperm is characterized by the absence of free-nuclear stage. The division of the primary endosperm nucleus and a few subsequent nuclear divisions are followed regularly by wall formation. In avocado, the wall formed after the first division of the primary endosperm nucleus is probably callosic. The occurrence of haustoria is a common feature of this type of endosperm; it is more varied than that in the nuclear endosperm. The haustoria may be micropylar or chalazal. Occasionally, both types of haustoria are present in the same plant. The first division of the primary endosperm nucleus is followed by a transverse wall resulting into two chambers of almost equal size. Divisions in the micropylar chamber are rapid and in all directions. The chalazal chamber divides transversely and at a comparatively slow rate. This results in a tail-like chalazal part attached to the more massive tissue at the micropylar end. Further divisions occur in the upper part of the tail and add to the endosperm tissue. The basal two or three cells of the tail elongate to form an haustorium which penetrates into the chalazal part of the nucellus.

Helobial endosperm:

                In this type of endosperm is restricted largely to the monocotyledons. The primary endosperm nucleus moves to the chalazal end of the embryo sac where it divides forming a larger micropylar chamber and a small chalazal chamber. In the micropylar chamber, as a rule, free-nuclear divisions and cell formation, if any, start at a much later stage. In the chalazal chamber the nucleus either remains undivided or divides only a few times. If latter is the situation, the divisions are usually free-nuclear. However, sometimes, as in Phylidrum lanuginosum, it may becomes cellular.  

Cereal endosperm:

In cereals the endosperm is of the free nuclear type. It is generally starchy and dry at maturity but in 30 out of the 16 grasses surveyed it remains liquid or soft even in mature seeds. Extensive work has been done on the structure, physiology and genetics of endosperm development in grasses because of its special structure and economic importance as the major source of calories in food and feed. Fully differentiated endosperm of cereals, such as maize and barley, consists of four distinct types of cells or tissues. Transfer cells (TCs), Embryo surrounding cells (ESCs), starchy endosperm cells (SECs) and Aleurone cells (ALCs).

Ruminate endosperm:

                Mature endosperm with any degree of irregularity and unevenness in its surface contour is called ruminate endosperm. Rumination starts at a late stage of endosperm development, and it may belong to anyone of the 3 categories described above. Ruminate endosperm is known to occur in about 32 families of angiosperms.

                Rumination is caused by the activity of the seed coat or the endosperm itself. In the former case, the irregularities on the inner surface of the seed of the seed coat may arise by:

1. Unequal radial elongation of anyone or the only layer of the seed coat.
2. Definite ingrowth or infoldings of the seed coat.

The second cause is more common and is found in the Annonaceae and Aristolochiaceae.

Functions of endosperm:

• It is the nutritive tissue for the developing embryo.
• In majority of angiosperms, the zygote divides only after the development of endosperm.
• Endosperm regulates the precise mode of embryo development.

 REFERENCE :

THE EMBROLOGY OF ANGISPERMS 6^th EDITION Author; SS BHOJWANI, SP BHATNAGAR ,PK DANTU.