The Seed: A Short Note!
It is customary to begin the study of Morphology with a study of the seed simply because the plant grows out of the seed.
Beginning Morphology with the seed is merely a convention and is not to be regarded as any attempt to solve the eternal question— whether the hen or the egg came first.
It is found convenient to begin with a description of the seed, then to study the seedling, then to study the different vegetative parts followed by the reproductive organs and finally to end with the formation of the fruit and the seed which brings us to the next generation and completes the life cycle of a flowering plant.
The Phanerogams or Spermatophytes are endowed with this peculiar but very common detachable vehicle of reproduction carrying the germ of a new individual which is known as the seed. Botanically speaking, a seed is to be defined as a mature integumented megasporangium. We do not get any integumented megasporangium among the Cryptogams, the highest development among which is exemplified by heterosporous plants like Selaginella and; some more complex fossil forms.
Among the Gymnosperms the megasporangium is usually covered by a single integument and this is known as the ovule. This ovule is naked, i.e., not enclosed in a fruit or the carpellary vessel, hence the name Gymnosperm. In the Angiosperms, the ovule or ovules are generally endowed with two integuments and are enclosed within a case formed by the carpel or carpels which is known as the fruit.
When these ovules become mature after fertilisation and some post-fertilisation changes (e.g., -hardening and drying up of the whole structure), it becomes the seed. It contains an embryo which passes a long or short period of dormancy and germinates when proper conditions are fulfilled. If the embryo within a seed gets killed, it fails to germinate under all conditions and the seed is said to have become inviable. Viability test is, therefore, very important in Agriculture and Horticulture as a test for assessing the usefulness of any seed sample.
In describing the parts of a seed, one has to proceed a little mechanically at this stage as the real nature of the different parts will become clear only when one has studied the development of the seed which is described towards the end of Morphology.
Every seed (general plan) has a seedcoav which is really the two dried up integuments, the testa (outer integument) and the tegmen (inner integument).
The testa and the tegmen may sometimes get so much fused together that they cannot be separated. -Inside the seedcoat is the kernel of the seed which contains the embryo or the rudimentary plant which is the most important part of the seed and may or may not be accompanied by some nutritive tissue (endosperm and perisperm).
An important part of the embryo is the cotyledon or the seed leaf which is a special type of leaf (discussed in connection with Leaf). Angiosperms are divided into the Monocotyledons and the Dicotyledons according to the number of cotyledons. Polycotyledonous types are known among Gymnosperms.
Some Angiospermic seeds do not show a proper differentiation of cotyledons but other characteristics put them along with either Monocotyledons or Dicotyledons. Both Dicotyledonous and Monocotyledonous plants may be Albuminous (Endospermic) or Exalbuminous (Exendospermic) according as whether the endosperm is present or not.