NCERT Class 11-Biology: Chapter –6 Anatomy of Flowering Plants Part 7

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Long Answer Type Questions

Question 1:

The arrangement of ovules within the ovary is known as placentation. What does the term placenta refer to? Draw various types of placentations in the flower as seen in T.S. and V.S.

Answer:

Placenta is a tissue which is flattened, cushion-like tissue. Through placenta ovule is attached to the wall of the ovary.

Placentations are of many types like marginal, axile, parental, basal, central and free central.

In Marginal Placentations the placenta forms a ridge along the ventral suture of the ovary and the ovules are borne on this ridge forming two rows, as in pea. This is found in monocarpellary, unilocular ovary.

In Axile Placentations the ovules are borne on central axis and the marginal of placenta grow inward and fuse, thus making a multilocular ovary, as in China rose, tomato, etc. Here numbers of locules are as many as the carpels.

In Parietal Placentations the ovules develop on the inner wall of the ovary or on peripheral part. Ovary is one chambered but it becomes two chambered due to the formation of a false septum known as replam, e.g., mustard, argemone. Here the placentae develop on the ovary wall and the number of placentae is equal to the number of carpels.

In Free Central Placentations the ovules are present on the central axis of ovary and septa is absent so ovary is unilocular, as in Dianthus and Primose. A single large swollen placenta arises from base of the ovary. It bears a number of ovules all over its surface.

In Basal Placentations the placenta develops at the base of ovary and a single ovule is attached to it, as in sunflower, marigold, etc.

Placenta function is to supply nutrients to the developing embryo.

Placenta function is to supply nutrients to the developing e …

Placenta Function is to Supply Nutrients

Question 2:

Deciduos plants shed their leaves during hot summer or in autumn. This process of shedding of leaves is called abscission. Apart from physiological changes what anatomical mechanism is involved in the abscission of leaves.

Answer:

Apart from physiological changes the other anatomical mechanism involved in the abscission of leaves are as follows:

i. Structural: In deciduous trees, an abscission zone also known as separation zone is formed at the base of the petiole. It is composed of top layer and bottom layer. The cells in the top layer have weak cell walls and the bottom layer expand in winter and break the cell walls of the top layer, this result in shedding of leaves.

ii. The loss of chlorophyll may also result in abscission process.

iii. Hormonal: Abscisic acid hormone stimulates abscission.

An abscission layer is formed at the junction of the leaf and stem, it prevents transport of substances to and from the leaves and causes death of leaf cell thereby shedding of leaves takes place.

Anatomically, the cells of abscission zone are thin-walled and without deposition of lignin or suberin. At the time of abscission, the middle lamella may dissolve between the cells of two middle layers but the primary wall remains intact. The middle lamella as well as the primary walls of the adjacent cells is dissolved. Ultimately the whole cells of middle layer found in the abscission layer gets dissolve completely. Thus, there is separation of plant organ, i.e., leaf from the plant, wherever there is rainfall or wind.

Question 3:

Is Pinus an evergreen tree? Comment.

Answer:

Evergreen trees are tree which shed their leaves throughout the year as they are covered with leaves all the time. These are persistent in all four seasons. In contrast to deciduous plants which completely lose their foliage during winter or dry season.

Pinus does not shed its leaves during a particular season and is always evergreen. Hence Pinus is considered as an evergreen tree belongs to gymnosperms.

So Pinus is evergreen, coniferous resinous tree.

The leaves in gymnosperms are well-adapted to withstand extremes of temperatures, humidity and wind.

In conifers, the needle-like leaves reduces the surface area. Their thick cuticle and sunken stomata also help to reduce water loss.

The flowering plants under conditions of extreme cold shed their leaves and become dormant. But Pinus due to the presence of bark, which is thick, needle-like leaves having sunken stomata, reduce the rate of transpiration. The cold areas are physiologically and physically dry due to scanty rainfall, precipitation as snow, decreased root absorption at low temperature and exposed habitats. But, Pinus is well adapted to such conditions. It continues to manufacture food during this period and grown to dominate other plants. This shows that Pinus is an evergreen tree. It does not shed its leaves, i.e., needles under any condition.

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