Development of Monocot Embryo of Angiosperms
Development of Monocot Embryo
The oospore divides to form a Proembryo which is a filament consisting of three cells, a large Basal Cell, a Middle Cell and a Terminal Cell. The basal cell is present towards micropylar end. It enlarges in size forming most of the suspensor. The middle cell divides repeatedly by a series of transverse and vertical divisions to [produce several superposed tiers of cells which produce in succession next to the basal cell a few Suspensor Cells. Radicle (root tip). Hypocotyl and Plumule (Stem-tip). The terminal cell divides repeatedly to produce a single Cotyledon which is terminal in monocots.
Endosperm of Angiosperms
The endosperm is the main source of food for the developing embryo and germinating seed. It is haploid ion gymnosperms and is a continuation of the female gametophyte. In angiosperms, it is formed as result of fusion of two polar nuclei and one of the male gametes forming Primary Endosperm Nucleus. Generally, the endosperm is triploid but haploid, tetraploid polyploid endosperms are also found.
In majority of cases, the polar nuclei fuse to form secondary nucleus before the union of the male gamete.
On the basis of method of formation of early stages of endosperms, the endosperm can be divided into three types.
- Nuclear Type: In this type of endosperm, the nuclear divisions are not accompanied by wall formation. The number of nuclei produced may be few (4-16) to many (several hundred) depending on the size of the embryo sac. The nuclei are pushed towards the periphery due to formation of a large central vacuole.
The wall formation generally progresses from the periphery towards the center or from the apex towards the base. For example, in Fritillaria.
- Cellular Type: In this type of endosperm, the first and the following divisions of the nucleus are accompanied by wall formation. In this way, the embryo-sac is divided into many chambers. The first wall is usually transverse.
- Helobial Type: In this type of endosperms, the first nuclear division is followed by a wall resulting in the formation of a Chalazal & Micropylar Chambers. In chalazal chamber the nucleus divides and the division is followed by wall formation. In micropylar chamber, the nucleus divides by free nuclear division, i.e., the division of the nucleus is not accompanied by wall formation. This type of endosperm is called helobial as it occurs frequently in the Helobiales.
Since the endosperm is meant for storage of food, its cells are usually thin walled, large and isodiametric. The stored food materials are starch, oils, fats, proteins, etc.
In members of the family Graminae, the outermost layer of the endosperm act as a cambium and produce a series of thin walled cells towards inner side. These cells become filled with starch and cease to function after sometime and are converted into Aleurone Layer. It is considered that this layer secretes an enzyme which assimilate the food stored in the endosperm and makes it available to the embryo.
In many seeds, no endosperm is seen at maturity, although it is formed at initial stages of embryo development. In these cases, during development of embryo the food stored in endosperm is utilized by the developing embryo and endosperm is completely exhausted. Such seeds are called Exalbuminous or Non-Endospermic, e.g., Gram, Pea, Bean, etc. In these seeds the food is stored in cotyledons. In other cases, the endosperm is not completely exhausted by the developing embryo. Such seeds are called Albuminous or Endospermic, e.g., Castor -oil seeds, Grasses.
In many cases, the endosperm-develops and completely fills up the nucellus space and no nucellus is found in the seed. However, in some cases, as in Nymphea the nucellus persists and develops into a nutritive tissue, called Perisperm. This is present in the seeds usually in the form of an apical cap.
In some families like Palmae, Rubaceae, the endosperm is irregular ridged and furrowed. This type of endosperm is termed as Ruminate Endosperm. The ruminations of the endosperm develop as a result of the outer tissue, which penetrates deeply and appears as dark wavy bands in the mature seeds, or sometimes it develops as a result of an unequal elongation of the cells of any one layer, or the only layer of the seed coat as in Passiflora.
In some angiosperms, there is a lack of uniformity in the tissue of the endosperm, such endosperm is termed as Mosaic Endosperm, e.g., in Zea mays (maize) the patches of two different colors are seen (white and yellow) forming a sort of irregular mosaic pattern.