are called as Scarification . ii. Impaction : In some seeds water and oxygen are unable to penetrate micropyle due to blockage by cork cells. These seeds are shaken vigorously to remove the plug which is called Impaction .
iii. Stratification : Seeds of rosaceous plants (Apple, Plum, Peach and Cherry) will not germinate until they have been exposed to well aerated, moist condition under low temperature ( o C to o C) for weeks to months. Such treatment is called Stratification . iv.
Alternating temperatures : Germination of some seeds is strongly promoted by alternating daily temperatures. An alternation of low and high temperature improves the germination of seeds. v. Light : The dormancy of photoblastic seeds can be broken by exposing them to red light.
. Senescence Plant life comprises some sequential events, viz : germination, juvenile stage, maturation, old age and death. Old age is called senescence in plants. Senescence refers to all collective, progressive and deteriorative processes which ultimately lead to complete loss of organization and function.
Unlike animals, plants continuously form new organs and older organs undergo a highly regulated senescence program to maximize nutrient export. The branch of botany which deals with ageing, abscission and senescence is called Phytogerontology . Types of Senescence Overall senescence Top senescence Deciduous senescence Progressive senescence Figure . : Different types of senescence in plants Leopold ( ) has recognised four types of senescence: i.
Overall senescence ii. Top senescence iii. Deciduous senescence iv. Progressive senescence i.
Overall senescence : This kind of senescence occurs in annual plants when entire plant gets affected and dies. Example: Wheat and Soybean. It also occurs in few perennials also. Example: Agave and Bamboo.
ii. Top senescence : It occurs in aerial parts of plants. It is common in perennials, underground and root system remains viable. Example: Banana and Gladiolus .
iii. Deciduous senescence : It is common in deciduous plants and occurs only in leaves of plants, bulk of the stem and root system remains alive. Example: Elm and Maple. iv.
Progressive senescence : This kind of