Class 12th Biology 2014 Set2 Delhi Board Paper Solution
Explain the ovarian and uterine events that occur during a menstrual cycle in a human female under the influence of pituitary and ovarian hormones respectively.
OR
(a) Why does endosperm development precede embryo development in angiosperm seeds? State the role of endosperm in mature albuminous seeds.
(b) Describe with the help of three labelled diagrams the different embryonic stages that include mature embryo of dicot plants.
Menstrual cycle is the reproductive cycle in all primates and begins at puberty (menarche). In human females, menstruation occurs once in 28 to 29 days. The cycle of events starting from one menstruation till the next one is called the menstrual cycle.
During the middle of the menstrual cycle, one ovum is released (ovulation). The cycle starts with the menstrual flow (3 to 5 days), caused due to the breakdown of the endometrium of the uterus. Blood vessels in liquid state are discharged, but this occurs only when the ovum is not fertilised.
It is followed by the follicular phase where the primary follicles mature into Graffian follicles.This causes the regeneration of the endometrium. These changes are brought about by ovarian and pituitary hormones. In this phase, the release of gonadotropins (LH and FSH) increases. This causes follicular growth and the growing follicles produce oestrogen. The LH and FSH are at their peak in the middle of the cycle (14th day) and cause the rupture of the Graffian follicles to release ovum. This phase is called the ovulatory phase.
The remains of the Graffian follicles get converted into the corpus luteum, which secretes progesterone for the maintenance of the endometrium. In the absence of fertilisation, the corpus luteum degenerates, thereby causing the disintegration of the endometrium and the start of a new cycle.
OR
(a) The development of endosperm precedes that of the embryo in angiosperm seeds, because the endosperm cells provide nutrition to the developing embryo. In mature albuminous seeds, endosperm exists as storage tissue. It stores starch and fat.
(b) Development of embryo: The zygote starts dividing and gives rise to proembryo. This proembryo further divides, forming a globular, heart-shaped and mature embryo.
Following are the steps that occur during the development of embryo:
The embryo develops at the micropylar end of the embryo sac where the zygote is situated. First, the zygote gives rise to proembryo and, then, to the globular, heart-shaped, mature embryo. A typical dicot embryo consists of an embryonal axis and two cotyledons. The portion of the embryonal axis above the level of cotyledons is called epicotyl. It contains the plumule (shoot tip). The portion below the axis is called hypocotyl. It contains the radicle (root tip). The root tip is covered by the root cap.Popular Questions of Class 12th biology
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