Seed germination

 The process of sprouting embryo (seed) or process by which organism grows, resulting in the emergence of radicle and plumule is called seed germination. For efficient germination, the embryo should be viable. Other factors like environmental factors or condition, how deep the seed is planted, water facility, temperature etc. are should be favourable for seed germination. All fully developed seed consists of viable embryo, food reserves, enzymes and 10-12% water within seed coat. Some seeds cannot produce radicle and plumule due to lack of embryos, these seeds are called empty seeds and never germinate. Dormant seeds are also do not germinate because they are subject to external environmental condition that stops initiation of cell growth. Due to external factors ( i.e. water, oxygen, light ,temperature ) and internal factors (i.e. impermiability, hard coat, hormones). Seed becomes dormant as a result it cannot initiate the metabolic processes and cell growth stops. During germination of seeds following processes takes place.
 ● Imbibition:- When water is plentiful, the seed becomes fully turgid.
 ●Activation of reserve materials:- With the help of water special proteins (enzymes) as well as food materials become active resulting in seed growth. Transportation of soluble metabolites to the embryo. 
●Sprouting of seed:- After cell division of seed, it grows a root first to access water underground and then shoots grows above the surface. Sprouted seed consists radicle and plumule. 
 Depending upon the place of cotyledons either cotyledons are pushed above ground or remain below, germination classified as:- 
1)Epigeal germination:- ‘Epi’ means upon and ‘ge’ means earth. So, epigeal germination is such type of germination in which cotyledons are pushed above the ground. It is due to the vigorous elongation of hy (the region just below the cotyledons). Cotyledons which are emerged above the surface turn green and leaf like structure which ultimately fall off due to finishing of stored food. Eg, beans, castor, sunflower, papaya. 2)Hypogeal germination:- ‘Hypo’ means below and ‘ge’ means earth. So, hypogeal germination is such type of germination in which cotyledon remains below the soil or just on the soil. It is due to elongation of epicotyl (the region just above the cotyledon). Cotyledon do not turn green and fall off ultimately. Eg, peas, gram, mango, rice, wheat, maize. Factors affecting seed germination • Temperature:- It is an important factor. Extremely low or cold temperature is not suitable for seed germination, seeds prefer suitable temperature (25-30°C) for seed germination. Seeds prefer higher temperatures for germination. The germination rate of seed is directly proportional to the rise in the temperature up to optimum. • Moisture or water:- Water is an essential factor for imbibition, enzyme activation which results in germination. • Soil and PH:- Seed require different types of mineral elements for germination which are obtained from soil. Also suitable soil PH for optimal function for enzyme. • Dormancy period:- Certain seeds undergo in resting period and do not germinate until favourable condition, which is known as dormant period. Presence of growth inhibitors like abscissic acid induce dormancy in seed. • Reserve food materials:- Food materials like CHO, proteins, lipids if are not accumulated sufficiently in seed, it may not germinate. • Longevity or viability of seed:- Seeds retain their viability for certain period which may vary from plant to plant or seed to seed. Non viable cannot germinate. • Poisons:- Mostly cyanide(CN-) cause poisoning and kill growing embryos. • Thinners or thickness of seed coat:- Seeds having thin seed coat can germinate faster than thicker seed coats. • Agronomic factors:- Defective crop husbandry, cloudy days during grain filling, inadequate plant protection, mechanical injury may affect the seed germination. 

 References 
 https://en.m.wikipedia.org/wiki/Germination
 http://www.tutorvista.com/biology/factors-affecting-germination-of-seeds?view=simple
 ^ "A Guide to Forest Seed Handling" . 

^ Siegel, S. M.; Rosen, L. A. (1962). "Effects of Reduced Oxygen Tension on Germination and Seedling Growth". Physiologia Plantarum . 15 (3): 437–444. doi :10.1111/ https://study.com/academy/lesson/what-is-seed-germination-definition-process-steps-factors.html  Singh, S.P. and Gaurav, S.S.(2014) “a text book of crop physiology.”
                                          Mukesh paneru,GAASC,IAAS