Gymnosperm Botany Grade 11 ( Pinus )

 GYMNOSPERMS

General Characters:

Ø They are naked seeded plants i.e. they do not have fruits and seeds are open.

 

Ø They are found from tropical to temperate region.

 

Ø Plant body is sporophyte.

 

Ø The sporophyte plant body is differentiated into root, stem and leaves.

 

Ø Roots are tap roots.

 

Ø Stem is branched and usually two types of branches are present i.e. branch of unlimited growth called long shoots and branch of limited growth called dwarf shoot.

 

Ø Leaves may be of one kind i.e. monomorphic or two kinds i.e. dimorphic (one is green leaf and another is minute scale leaf).

 

Ø Plants has vascular tissues i.e. xylem and phloem.

 

Ø Xylem lacks vessels and phloem lacks companion cells (except Ephedra and Gnetum).

 

Ø Pollination is anemophily and direct.

 

Ø Endosperm is haploid.

 

Ø Double fertilization and triple fusion is absent.

 

Ø Polyembryony is common.

 

PINUS

   

 

Habit: Pinus is xerophytic monoecious plant.

 

Morphology: Plant body is sporophyte differentiated into root, stem and leaves.

 

Root: Root is tap root. Root has symbiotic relationship with fungi called mycorrhiza.

 

Stem: Stem is erect, branched and woody. Stem bears two types of branches i.e. long shoot (which arise from main stem and grows indefinitely) and dwarf shoot (which arise from long shoot and grows for a short time). Long shoot bears only scale leafs while dwarf shoot bears scale as well as foliage leaf.

 

Leaves: Pinus is dimorphic i.e. possess two types of leaves: scale leaves and foliage leaves. Scale leaves are thin, brown, flattened and minute structures which fall off with maturity of branches while foliage leaves are long, needle like and green. The dwarf shoot bearing foliage leaves is called spur.

 

Reproduction:

Pinus is monoecious and bears male and female cones on different branches of same plant. Male cone develops in cluster (15-140) on base of long shoot. They arise from axils of scale leaf and develops later than male cone. Female cones grow very slowly thus female cones of different ages may be seen in acropetal succession in the long shoot.

 

Male cone:

 

Each male cone is small and oval shaped. It arises in clusters from the axis of scale leaves on dwarf shoot. The male cone has a central axis on which 60-150 microsporophylls are spirally arranged around the axis. A single microsporophyll is a membranous stalked structure with a distal expanded roughly triangular sterile part called apophysis.

Each microsporophyll bears two sac-like microsporangia on the abaxial surface. A mature microsporangium consists of a multilayered wall, tapetum and microspore mother cells. Each microspore mother cell by meiotic division produces four microspores or pollen grains.

 

The pollen grains are boat-shaped with monosulcate apertures and are bounded by two concentric wall layers: the outer thick exine and the inner thin intine. The exine on the lateral sides of the pollen is expanded to form two wings (sacci). Pinus is wind-pollinated (anemophilous).

 

 

Female cones

 

They are produced in pairs or in clusters in the axil of the scale leaves. The female cones mature very slowly. The fully matured third year cone is much larger (15-60 cm in length), woody, loose and brown in colour. Here megasporophylls are separated from each other due to the elongation of the cone axis. The female cone is composed of a central axis on which 80-90 megasporophylls, axillary to bract scale/scale leaves, are arrange spirally.

 

The bract scale and ovuliferous scale thus form a seed-scale complex. A single megasporaphyll consists of two types

of scales:

 

(a) a large woody ovuliferous scale or seminiferous scale bearing two ovules on the adaxial surface, and

(b) a bract scale or cone scale on the abaxial surface.

 

Initially, the ovuliferous scale is much smaller than that of bract scale, but after pollination it becomes larger than the bract scale. The ovuliferous scale is a thick, large, woody, roughly triangular and brownish structure. Its upper thick exposed part is known as apophysis.

 

The ovules of Pinus are anatropous, unitegmic and crassinucellate. The single integument is free from the nucellus except at the chalazal end. There is a fairly broad micropylar tube which becomes inwardly curved during prepollination stages and becomes outwardly curved at the time of pollination.

 

Fertilization

The fertilization takes place after one year of pollination. One of the male nuclei fuses with the egg cell and thus a zygote is formed.

Grade-11 Classification of elements and periodicity in properties

 Classification of elements and periodicity in properties

 

Why do we need to classify elements?

Before the 19^th century only a few elements were known so they could be easily studied but with time many more elements were discovered and their compounds were prepared hands it was felt that these elements should be classified into a few groups to make their studies systematic and easier it is expected that a systematic classification will help us in organizing the knowledge and predicting new elements for undertaking further studies

Dobereiner’s triad

Dobereiner’s triad:  In 1829, a German scientist John Dobereiner classified certain elements in a Group of three called triad which had similar chemical properties. Dobereiner’s triad states that “when the elements in a triad were arranged in the order of increasing atomic mass is the atomic mass of the middle element was found to be approximately equal to the arithmetic mean of the other two elements.”

Limitations of Dobereiner’s Triad classification: Dobereiner’s method of classification could arrange only a limited number of elements the idea of trades could not be applied to all elements

Newlands law of octaves

In 1864 John Newlands an English chemist and a musician arranged the elements known at that time in order of increasing atomic mass which state that the properties of each element resembled those of the 8^th element before it and off the 8^th element following it in other words he found that the properties of the elements were repeated at every 8^th element like the death note of an octave in music Newlands called this as the law of octaves

The drawbacks of Newland's classification are he failed badly with dealing with heavier elements beyond calcium. The idea of octaves could not be held for noble gases.

Lothar Meyer's atomic volume curve

In 1869 a German chemist Julius Lothar Meyer plotted a graph of atomic volume versus the atomic mass of various elements he noticed that the elements with similar properties occupied similar positions on the curve for example lithium, sodium, potassium, rubidium, and cesium occupied the big position. It is the first classification of elements which had a definite pattern and periodicity in properties of elements

 

Mendeleev’s classification

The Russian chemist Dimitri Mendeleev arrange the elements in the increasing order of atomic mass. He observed that the elements with similar properties appeared at regular intervals the Mendeleev spirit law states that, “the properties of the elements are a periodic function of their atomic mass.”

In other words, according to this law when the elements are arranged in increasing order of their atomic masses the elements with similar properties are repeated at regular intervals

Mendeleev periodic table:

The elements were known at that time and are arranged in increasing order after atomic mass in a table known as the periodic table. Elements having similar properties were placed in vertical columns called groups and horizontal rows were called periods. When the order of increasing atomic mass was not rigidly followed then he left gaps in his periodic table and even predicted the properties of such elements.

For example, Mendeleev had predicted the properties of elements with an atomic mass of 72 which he named eka-silicon.

Modern Periodic Table

Modern Period Table was designed by Henry Moseley in 1913 A.D. Modern Periodic Table states that “the properties of the elements are the periodic function of their atomic number.”

In Modern Period table: The regular repetition in elements occurs due to periodicity in the outer electronic configuration of elements. Isotope does not need a separate position to the classification of an element based on the classification of elements. The left portion of the periodic table includes highly reactive metals, the right includes highly reactive non-metals and the middle-portion includes transition metals. Elements were divided into s, p, d, and f blocks.