Study of Different Families

Study of Different Families

 

INTRODUCTION

 

A British systematic botanist J. Hutchinson published his work, The Families of Flowering Plants in 1926 on dicotyle-dons and in 1934 on monocotyledons. Hutchinson made it clear that the plants with sepals and petals are more primitive than the plants without petals and sepals on the assumption that free parts are more primitive than fused ones. He also believed that spiral arrangement of floral parts, numerous free stamens and hermaphrodite flowers are more primitive than unisexual flowers with fused stamens. He considered monochlamydous plants as more advanced than dicotyledons. Hutchinson’s system indicates the concept of phylogenetic classification and seems to be an advanced step over the Bentham and Hooker system of classification. Hutchinson accepted the older view of woody and herbaceous plants, and fundamentally called them as Lignosae and Herbaceae. He revised the scheme of classification in 1959. He has divided the flowering plants into two phyla: phylum I—Gymnospermae (not elaborated by him) and phylum II—Angiospermae. The latter are divided into two sub-phyla: sub-phylum I—Dicotyledons and sub-phylum II—Monocotyledons.

 

The division of angiosperms into these two large classes is based on the following factors:

 

1)    In dicotyledons, the embryo bears two cotyledons, and in monocotyledons, it bears only one.

 

2)    In dicotyledons, the primary root persists and gives rise to the tap root, while in monocotyledons, the primary root soon perishes and is replaced by a cluster of adventitious (fibrous) roots.

 

3)    As a rule, venation is reticulate in dicotyledons and parallel in monocotyledons. Among monocotyledons, aroids, sarsaparilla (Smilax) and yams (Dioscorea), however, show reticulate venation, and among dicoty-ledons, Alexandrian laurel (Calophyllum) shows parallel venation. Further, in dicotyledons, the veinlets end freely in the mesophyll of the leaf, whereas in mono-cotyledons, veins or veinlets do not end freely.

 

4)    The dicotyledonous flower usually has a pentamerous symmetry, sometimes tetramerous (as in Cruciferae and Rubiaceae), while the monocotyledonous flower has a trimerous symmetry.

 

5)    In the dicotyledonous stem, the vascular bundles are arranged in a ring and are collateral and open, i.e. they contain a strip of cambium which gives rise to secondary growth. In the monocotyledonous stem, however, the bundles are scattered in the ground tissue and are collateral and closed. Hence, there is no secondary growth (with but few exceptions). Also the bundles are more numerous in monocotyledons than in dicotyledons. Further, they are more or less oval in monocotyledons and wedge shaped in dicotyledons.

 

6)    In the dicotyledonous root, the number of xylem bundles varies from 2 to 6, seldom more, but in the monocotyledonous root there are many, seldom a limited number (5–8). It may also be noted that the cambium soon makes its appearance in the dicotyle-donous root as a secondary meristem and gives rise to secondary growth, but in the monocotyledonous root, the presence of cambium is rare. Hence, there is no secondary growth.

 

Floral Diagram

 

The number of parts of a flower, their general structure, arrangement and the relation they bear to one another (aestivation), adhesion, cohesion, and position with respect to the mother axis may be represented by a diagram known as the floral diagram. The floral diagram is the ground plan of a flower. In the diagram, the calyx lies outermost, the corolla internal to the calyx, the androecium in the middle, and the gynoecium in the centre. Adhesion and cohesion of members of different whorls may also be shown clearly by connecting the respective parts with lines. The black dot on the top represents the position of the mother axis (not the pedicel), which bears the flower. The axis lies behind the flower and, therefore, the side of the flower nearest to the axis is called the posterior side, and the other side away from the axis the anterior side. The floral characteristics of species may be well represented by a floral diagram, whereas more than one diagram may be necessary to represent a genus or family.

 

Floral Formula

 

The different whorls of a flower, their number, cohesion and adhesion may be represented by a formula known as the floral formula. In the floral formula, K stands for calyx, C for corolla, P for perianth, A for androecium and G for gynoecium. The figures following the letters K, C, P, A and G indicate the number of parts of those whorls. Cohesion of a whorl is shown by enclosing the figure within brackets, and adhesion is shown by a line drawn on top of the two whorls concerned. In the case of the gynoecium, the position of the ovary is shown by a line drawn above or below G on the figure. If the ovary is superior, the line should be below it; and if inferior, the line should be on top. Thus, all the parts of a flower are represented in a general way by a floral formula.

 

Besides, some symbols are used to represent certain features of flowers. Thus ♂ represents male, ♀ female, H hermaphrodite, ♂♀ dioecious, ♂-♀ monoecious, ♂ ♀ H polygamous, ⊕ actinomorphic, ·׀·zygomorphic, ∞ indefinite number of parts, etc.

 

Features used in descriptions of Angiospermic plants:

 

·  Habitat: Natural abode of the plant.

 

·  Habit: Herb (erect, prostrate, decumbent, diffuse, trailing, twining or climbing), shrub (erect, straggling, twining or climbing), tree or any other peculiarity in the habit.

·  Root: Nature of the foot; any special form.

 

·  Stem: Kind of stem—herbaceous or woody; cylindrical or angular; hairy or smooth; jointed or not; hollow or solid; erect, prostrate, twining or climbing; nature of modification, if any.

 

·  Leaf: Arrangement—whether alternate, opposite (super-posed or decussate) or whorled; stipulate or exstipulate; nature of the stipules, if present, simple or compound; nature of the compound leaf and the number of leaflets; shape and size; hairy or smooth; deciduous or persistent: venation; margin; apex; and petiole.

 

·  Inflorescence: type of inflorescence.

 

·  Flower: sessile or stalked; complete or incomplete; unisexual or bisexual; regular, zygomorphic, or irregu-lar; hypogynous, epigynous or perigynous; bracteate or ebracteate; nature of bracts and bracteoles, if present; shape, colour and size of the flower.

·  Calyx: polysepalous or gamosepalous; number of sepals or lobes; superior or inferior; aestivation; shape, size and colour.

 

·  Corolla: polypetalous or gamopetalous; number of petals or lobes; superior or inferior; aestivation; shape, size colour and scent; corona or any special feature. (When there is not much difference between the calyx and the corolla, the term perianth should be used. It may be sepaloid or petaloid, polyphyllous or gamophyllous, or free or epiphyllous).

 

·  Androecium: number of stamens—definite (10 or less) or indefinite (more than 10); free or united; nature of cohesion—monadelphous, diadelphous, polyadelphous, syngenesious or synandrous; nature of adhesion—epi-petalous or gynandrous, or any special feature; whether alternating with the petals (or corolla lobes) or opposite them. Length of stamens—general length; inserted or exerted; didynamous or tetradynamous; position of stamens—hypogynous, perigynous or epigynous; attach-ment of the anther and its dehiscence; anther lobes or appendages, if any,

 

·  Gynoecium or pistil: number of carpels; syncarpous or apocarpous; nature of style—long or short; stigmas— simple, lobed or branched; their number and nature— smooth or papillose; ovary—superior or inferior; number of lobes; number of chambers (loculi); nature of pla-centation; number and form of ovules in each loculus of the ovary.

 

·  Fruit: kind of fruit.

 

·  Seeds: number of seeds in the fruit; shape and size; albuminous or exalbuminous; nature of endosperm, if present.