Morphological Characters of Angiosperms

The following points highlight the four main Morphological Characters of Angiosperms:- 1. Mode of Living 2. Gross Structure of Embryos and Seedlings 3. Types of Germination 4. Habits of Plants.

Angiosperms: Morphological Character # 1. Mode of Living:

Majority of the angiospermic plants are autotrophic but there are also many heterotrophic plants. Some of the heterotrophic angiosperms depend completely on host plants, but some other plants are partially dependent on the host plant or collect a portion of their nutrients from other types: of organisms or from decaying organic substances.

A. Autotrophic Angiosperms:

These plants have definite structures for collecting raw materials (e.g. O₂, H₂O, CO₂, Minerals, light etc.) for the synthesis of food-substances. These plants have well-developed organs for growth and reproduction (Examples – Solamun villosum, Fig. 9.1-A; Pistia stratiotes Fig. 9.1-B).

Some autotrophic plants grow on stems and branches of other larger plants but are not dependent on the hosts for their nutrition. These green plants often have two types of roots, one type for holding (clinging root) and the other for absorbing moisture from the atmosphere (aerial root). Aerial roots are sometimes provided with velamen

e.g. Vanda tessellate, Begonia gemmipara, Hymenopogon parasiticus, Pothos scandens, etc.

B. Heterotrophic Angiosperms:

These plants are completely or partially dependent on other plants (host) or decaying organic substances for their nutrition. The former are called parasites and the latter saprophytes.

1. Total parasites:

Non-green plants, attached to the host plant by means of haustoria and having no connection with the soil.

(a) Total stems parasites: Cuscuta reflexa (Fig 9.1-C), Arceuthobium minutissimum.

(b) Total root parasite: Orobanche indica (Fig 9.1-D) Balanophora dioica, Aeginetia indica.

2. Partial parasites:

Green-plants, attached with the host plant by mean of haustoria. The green colour indicates their capability of photosynthesis.

(a) Partial stems parasites:

Haustoria present but are devoid of any normal root, e.g. Dendrophthoe falcata (Fig 9.2-B) Viscum articulatum (Fig 9.2-C) Cassytha filiformis.

(b) Partial root parasites:

Haustoria present but are also provided with normal roots Striga lutea (Fig 9.2-A) Santalum album (young plants).

3. Saprophytic angiosperms:

These non-green plants draw their nutrition from decaying plant bodies, roots slender and leaves scaly, e.g. Monotropa uniflora (Fig 9.2-E) Dedymoplexis pallens, Galeola lindleyana.

4. Insectivorous plants:

A group of nearly autotrophic plants, but generally captures some small animals by some specialised organs, mainly modification of leaves, and digests the soft portion of the preys.

Following are some common examples:

A. Drosera burmanii:

A small rosette plant, leaves petiolates, lamina Spathulate to orbicular, provided with many long tentacles on the margins, tentacles gland tipped; flowers on long scapes (Fig 9.2-F).

B. Utricularia stellaris:

A free floating plant, roots absent; leaves much dissected, some dissected leaflets modified into pear shaped bladders. Under simple microscope these bladders show an opening which is surrounded by many branched and un-branched bristles. When splitted and cotton-blue stained, the bladders show some stalked digestive glands with 2 – 4 radiating arms on the wall of inner surface (Fig 9.2 G, H, I).

C. Nepenthes khasiana:

Leaves long, petiolate, upper portion of lamina modified into a stalked pitcher ; pitchers oblong, mouth oblique, rim of the mouth slippery; upper side of the mouth provided with a small lamina and broad and coloured lid (Fig 9.2-J).

Angiosperms: Morphological Character # 2. Gross Structure of Embryos and Seedlings:

A. Dicotyledonous Embryos:

A well-differentiated dicotyledonous embryo consists of the following parts:

(a) Two cotyledons,

(b) Tigellum,

(c) Plumule and

(d) Redicle.

(i) Embryo of Pisum sativum (Fig 9.3-A):

Removal of the seed coat with the help of nails will expose the embryo. Cotyledons half-spherical, very thick, tigellum small and straight; plumule and radicle easily recognisable. There is no endosperm tissue surrounding the embryo.

(ii) Embryo of Ricinus communis (Fig. 9.3-B):

Manual removal of splitted endosperm will show the embryo. The embryo remains embedded within the endosperm-tissue; straight cotyledons are broad and thin; veins and veinlets prominent; tigellum small; plumule and radicle well differentiated.

Any leguminous seed can be used to demonstrate its exalbuminous nature and thick cotyledons. Seeds of Artocarpus heterophylla and Trapa bispinosa bear unequal and thick cotyledons. Albuminous seeds of Carica papaya, Gossypium arboreum, Mirabilis jalapa, etc. can be used to demonstrate thin cotyledons.

B. Monocotyledonous Embryos:

Monocotyledonous embryos consist of;

(a) Only one cotyledon,

(b) Radicle and plumule, but a tigellum is not well recognisable.

The structure of grass-embryo is somewhat different from other types.

(i) Embryo of Zea mays:

(Fig 9.3-E) L.S. of a water-soaked seed, stained with iodine, shows the embryo towards the base of caryopsis: The upper part of the embryo is a shield-shaped structure called scutellum. On the lower portion of the scutellum there is a short axis.

On the upper side of this axis there is a plumule, covered by coleoptile which contains a number of small unequal leaves of which the outermost is the largest. The radicle is present on the lower portion of the axis; remains covered by a coleorhiza, and are with a distinct root-cap zone at the tip. The seed is albuminous and the embryonic leaves are thin.

(ii) Embryo of Allium cepa:

(Fig 9.3-C) The embryo will come out of the seed with little pressure on the seed surface by nail or the handle of a needle. The curved embryo remains embedded within the endosperm. The narrower side is radicle and broader side is scutellum. Plumule is undifferentiated at this stage.

(iii) Embryo of Vallisneria spiralis:

(Fig 9.3- D) The seed is exalbuminous with a large and folded embryo. The narrower side is the radicle and the thick side is cotyledon. Plumule is undifferentiated.

C. Dicotyledonous Seedlings:

The modes of germination of dicotyledonous seeds may be epigeal or hypogeal and the structure of seedlings varies accordingly.

(i) Seedling of Brassica juncea (Fig 9.4-A):

The mode of germination of this exalbuminous seed is epigeal.

The erect seedling shows:

(a) Young taproot with small lateral branches;

(b) Cotyledons obcordate, green and function as first assimilatory leaves of the plant;

(c) The plumule produces normal leaves in alternate phyllotaxy.

The seedlings of Cucurbita maxima, Tamarindus indica and many other members of Cruciferae, Cucurbitaceae, Leguminosae, etc. show a similar structure.

(ii) Seedling of Pisum sativum (Fig 9.4-B):

The mode of germination of its exalbuminous seed is hypogeal. The small 3-leaved seedling shows:

(a) A young tap-root with small lateral branches,

(b) The cotyledons are attached at the cotyledonary node;

(c) Seed coat encloses the cotyledons

(d) First plumular leaf is developed into a prophyll with reduced stipules,

(e) Second leaf produces some leaflets,

(f) Third leaf is normal with foliaceous stipules, leaflets and tendril.

The seedlings of Viciafaba, Phaseolus mungo, Mangifera indica, Cicer arietinum, etc. also show more or less similar structures.

(iii) Seedling of Ricinus communis (Fig 9.4-C):

The mode of germination of the albuminous seed of castor is epigeal.

A four-leaved seedling shows:

(a) Lower two leaves are thin and green cotyledons with prominent veins and veinlets;

(b) Plumule has produced the first pair of leaves which are small and normal in shape (i.e. palmately lobed),

(c) Propylls are absent;

(d) Tap root normal with numerous lateral branches.

Seedling of almost all members of Euphorbiaceae is more or less of the same type.

D. Monocotyledonous Seedlings:

Like dicotyledonous seedlings, seed of monocotyledonous plants also show epigeal or hypogeal mode of germination and accordingly, the structure of seedling also varies:

(i) Seedling of Allium cepa (Fig 9.4-D):

The mode of germination is epigeal.

A seedling consists of:

(a) One long, narrow tubular cotyledon (scutellum) which in turn is divisible into two distinct parts: the tip remains inside the seed and the lower portion is nearly erect and green;

(b) Plumule comes out piercing the base of the cotyledon;

(c) Radicle is un-branched and is of restricted growth;

(d) A number of adventitious roots of different sizes develop from the base of radicle.

(ii) Seedling of Zea mays (Fig 9.4-E):

The mode of germination is hypogeal.

A seedling consists of:

(a) Elongated, radicle and with a few lateral branches;

(b) 2 – 4 seminal roots developed from the base of the radicle;

(c) Cotyledon (scutellum) remains inside the seed;

(d) Mesocotyle region;

(e) A few adventitious roots developed from the node above the mesocotyle;

(f) Short, tubular and colourless plumule.

First green leaf has emerged by piercing the tip of the coleoptile.

Seedlings of Oryza saliva, Avena saliva, Triticum aestivum, and other members of Gramineae are more or less similar.

(iii) Seedling of Phoenix sylvestris (Fig 9.4-F):

Mode of germination is hypogeal. Unlike other seedlings, the entire embryo of date-palm (and many other members of Palmae) is pushed deep into the soil with the elongation of the lower portion of a rigid and non-green cotyledon. The terminal portion of cotyledon remains within the seed.

Coleorhiza is slightly elongated and radicle comes out piercing it. Radicle forms the first root and is provided with numerous lateral branches. 2 – 4 adventitious roots are developed from the base of the radicle. First green leaf emerges, by piercing the cotyledonary sheath.

E. Viviparous-seedling of Ceriops roxburghiana (Fig 9.4-G):

‘Vivipary’ is a special mode of germination found mainly in the plants of mangrove vegetation. One such seedling at the time of detachment from the mother plant shows small gray fruit from which a long and green radicle has come out. Its thickest region is situated a little above the tip and this taper towards the tip. Cotyledons and the plumule have still remained within the fruit (or seed).

Angiosperms: Morphological Character # 3. Types of Germination:

Germination is mainly of two types: Epigeal and Hypogeal. There is not much of difference in the mode of germination in between albuminous and exalbuminous type of seeds.

For the study of germination, different type’s seeds are put in moist sand or saw-dust in an earthen pot for a few days when some seedlings emerge and these can be demonstrated. Some seeds, showing different stages of germination, may be taken out cautiously from the seed-bed, which will help to understand the process perfectly.

A. Epigeal mode of Germination:

(i) Germination of Cucurbita maxima Seeds (Fig 9.5-A):

The following stages are found to complete the germination process of this exalbuminous seed:

(i) Radicle comes out first and enters into the deeper layer of soil;

(ii) Radicle produces some lateral root;

(iii) Hypocotyl region elongates to form a peg-like structure;

(iv) Gradually the lower portion of the cotyledons come out of the seed-coat;

(v) With the erection of the hypocotyl, cotyledons come out of soil bearing the seed-coat at the tip, which generally is stripped off very soon and the green cotyledons spread;

(vi) Elongation of epicotyl starts after this.

(ii) Germination of Ricinus communis Seeds (Fig 9.5-B):

The following stages are found to complete the germination process of this albuminous seed:

(i) On swelling, the seed-coat softens and cracks near the caruncle;

(ii) Radicle comes out and enters the deeper layer of soil;

(iii) Lateral roots develop very soon;

(iv) Hypocotyl region starts elongation forming a peg like structure;

(v) The cotyledons gradually come out of the broken seed-coat and the final remnants of endosperm;

(vi) Cotyledons turn green very soon;

(vii) Epicotyl starts elongation.

B. Hypogeal mode of Germination:

1. Germination of Cicer arietinum Seeds (Fig 9.5-C):

The following stages are found to complete the germination process of this exalbuminous seed:

(i) Radicle comes out first by piercing the softened seed-coat and enters the deeper layers of soil;

(ii) Radicle produces some lateral roots;

(iii) Hypocotyl does not elongate, so the co­tyledons never come out of the soil;

(iv) Epicotyl grows rapidly and brings the plumule outside the soil, i.e. initiates the growth of a normal aerial stem.

2. Germination of Oryza sativa Seeds: (Fig 9.5-D):

It is an albuminous seed and the following stages are found to complete its germination:

(i) Coleorhiza pierces the seed

(ii) Fruit wall; radicle enters the soil after splitting open the coleorhiza;

(iii) Radicle produces some lateral roots;

(iv) Two seminal roots grow from the base of the radicle;

(v) Coleoptile comes out and grows erect;

(vi) Plumule comes out by splitting open the coleoptile and produces the first green leaf;

(vii) Mesocotyl elongates and a number of adventitious roots are produced from the cotyledonary node.

Angiosperms: Morphological Character # 4. Habit of Plants:

Habits (or forms of stem) are not the same in different plants.

Plant-habits can be broadly divided into two groups:

A. Erect Forms:

Stem stands erect on the soil and roots are f produced only from their bases (except some special cases like Ficus benghalensis, F. elastica, etc.)

Plants of erect habit can be divided into three groups according to the form of the main stem, branching pattern and strength of the stem:

1. Herb:

Stem soft, un-branched or branched; generally short-lived.

Herbs are again of different forms as described below:

(a) Ephemerals:

Plants which complete their life cycle within a few weeks or a season; e.g. Anagalis arvensis (Fig 9.8-C), Lathyrus aphaca, etc.

(b) Annual Herb:

Plants survive for an year only, e.g. Lindenbergia indica (Fig 9.6-A) Solanum villosum, Ageratum conyzoides, etc.

(c) Biannual Herb:

These plants require two years (or at least more than one year) to complete their life cycle: e.g. Bryophylum calycinum (Fig 9.6-B). These plants generally flowers in the second winter of its life. Biannuals are not common and are not available in our locality except in very cold areas. Oenothera biennis is a good example.

(d) Perennial Herb:

Generally plants with underground parts which survive more than two years ar6 considered under this category. These plants produce annual aerial stems during, favourable season (e.g. Hedychium coronarium, (Fig 9.6-C), or branches bear a pseudo-stem which completes the life cycle (e g. Musa bulbisiana.)

(e) Suffrutescent Herbs:

These are also perennial herbs in which the aerial branches die during unfavorable season but their bases survive near the ground and produces new branches during favourable season, e.g. Desmodium gangeticum, Sida rhombifolia (Fig 9.6-D) etc.

(f) Rosette Herbs:

These may be annual (e.g. Blumea lacera, Capsella bursa-pastoris, (Fig 9.6-E) or perennials (e.g. Elephantophus scaler, Bellis perenis. Ainslea aptera etc. These plants produce radical leaves (i.e. internodes remain suppressed and all leaves are aggregated at the top of the root), at least during its vegetative growth period. These plants are acaulescent and produce scape during the flowering season (e.g. Allium capa, Drosera burmanii, etc.).

2. Shrubs:

These are larger than herbs, with woody stem and produce branches from their base (e.g. Solanum torvum, Hibiscus rosa-sinensis. Polyalthia suberosa etc.

Under shrubs are small shrubs with less woody stem e.g. Solanum indicum, Cassia sophera.

Super-shrubs on the other hand, are very large shrubs but do not possess any well-defined trunk to be considered as trees e.g. Edgwarthia gardnerii. Tabernamontana coronaria, Anona reticulata, etc.

3. Trees:

These are very large plants with distinct and woody trunks and produce branches either high above the trunk (e.g. Polyalthia longifolia) or from the top of the trunk (e.g. Tectona grandis).

The following types of stem are generally recognised:

(a) Caudex:

Usually do not possess any branch on the stem e.g. Cocos nucifera (Fig 9.7-A), Ravenala medagascariensis, etc. or rarely with very few branches (e.g. Hyphaene thebiaca). This habit is also known as monopodial.

(b) Ex-current:

In this type of plant the main stem maintains an indefinite apical growth and also produces lateral branches. So, branches develop from the side of the trunk, grows nearly parallel to the ground and the lowermost branches are largest giving the plant a stalked and elongated-triangular shape, e.g. Polyalthia longifolia (Fig 9.7-B), Eucalyptus globosus, Casuarina equisetifolia, etc.

(c) Deliquescent:

The apical bud of the main stem dies early or becomes weaker than the apical buds of lateral branches. The growth of primary, secondary, tertiary, etc. branches also ceases very soon giving the tree a spreading habit, e.g. Albizia labeck, Tectona grandis, Ficus religiosa (Fig 9.7-C), Delonix regia, etc.

(d) Tree Culm:

The stem is fistular (i.e. with solid nodes and hollow internodes) like normal grass stem but are very tall with ex-current growth pattern and do not further thicken by secondary growth e.g. Bambusa tulda.

B. Weak Forms:

The stems of these plants are weak and are not capable of keeping the entire plant body in erect state. These plants either grow on ground-surface or climb up by various means on some support.

These are primarily placed into four groups:

(i) Creepers;

(ii) Trailers;

(iii) Climbers and

(iv) Thalloid plants.

(i) Creepers:

These are all herbaceous plants i.e. with soft stem. Branches of the plants creep on soil surface get rooted at the nodes, produce lateral branches which in turn again get rooted like the primary branches and the process continues for the branches of further generations.

However, each such node is regarded as a separate plant. This type of habit is generally known as runner, e.g. Cynodon dactylon (Fig 9.8-A), Hydrocotyle himalaica, Oxalis corniculata, Centella asiatica, etc.

(ii) Trailers:

These are also herbaceous and prostrate plants but do not get rooted at each node. Old stems sometimes get rooted throughout its length (i.e. both at nodes and internodes).

These are again of two types:

(a) Procumbent Trailers:

Branches grow flat on the soil surface, e.g. Basella rubra (Fig 9.8-B).

(b) Decumbent Trailers:

Young branches grow more or less erect but their lower portions remain prostate, e.g. Tridax procumbens, Mazus japonicus, Leucus biflora, etc.

(iii) Climbers:

The weak stems of these plants always tend to grow upwards. For this purpose they need the support of some other structure, like the stems of other plants, rocks or wall surfaces, etc. Different plants use different parts of their body as climbing organ.

Depending on the nature of climbing organ, climbers can be of the following types:

(a) Twiners

(b) Lianas

(c) Scramblers

(d) Ramblers

(e) Hook climbers

(f) Root-climbers

(g) Adhesive climbers

(h) Tendril climbers

(a) Twiners:

The weak stem of some plants tend and coil like the wire in a spring surrounding a narrow support (e.g. branch of a strong plant) and grows upwards.

According to the direction of bending of the stem tips, these are again of two types:

(i) Dextrose – When the direction of coiling is clockwise e.g. Mikania cardata (Fig 9.9-A), Phaseolus mungo, Asparagus racemosus, etc.

(ii) Sinistrose – When the direction of coiling is anticlockwise, e.g. Clitoria ternatea, Ipomoea quinata (Fig 9.9-B) etc.

(b) Lianas:

These plants also grow up like twiners but are much larger plants and climb up to the top of the tallest trees in the forests. Their stems become very woody in due course of time e.g. Bauhinia vahlii, Hiptage madhobolata, Wisteria chinensis, Mucuna macrocarpa, etc.

(c) Scramblers:

These plants scramble upon other plants with the help of different types of spinous structures present on the stem (e.g. Rubus paniculatus, Rosa serisea, Lentana acuileata, etc.) on leaves and leaf-sheaths (Calamus rotung), prickles are also present on a whip-like specialised struoture (developed on the leaf-sheath), as modified stipules (Zyzyphus nummularius, Fig 9.9-C).

(d) Ramblers:

There is no specialised climbing organ in these plants, but they send some semi-erect tall branches through the branches of some strong erect plant and rest on the latter, gradually thicken and again send branches upward. Sometimes lateral small branches developed on these semi-erect branches help to keep them close together, which make them stronger and can move further upwards e.g. Quisqualis indica, Plectocomia himalaica, Rubus elepticus, etc.

(e) Hook-climbers:

These plants produce some specialised hook-like structures which anchor on the support and help them to climb up. In Artabotrys odoratissima (Fig 9.9-D) a branch of the peduncle forms the hook, and the same is produced by a small lateral branch in Vintillago moderaspatana.

(f) Root-climbers:

These plants climb with the help of some adventitious roots. These roots are produced either from the nodes only (Piper betel, Fig 9.9-E) or from internodes also (Scindapsus officinale, Fig 9.9-F; Rhaphidophora glauca, etc.)

(g) Adhesive climbers:

Some adhesive discs produced on the adventitious roots or other organs of the plant help these plants to climb-up a flat surface e.g. Hedera helix, Ficus hederacea (Fig 9.9- G), Cayratia trifolia, etc.

(h) Tendril climbers:

There are a large number of climbers which produce a special spring-like coiled structure, known as tendril, which helps them to hold the support strongly, making the climbing process very easy.

Depending on the nature of organs modified into tendrils, these plants are generally considered into the following types:

(i) Stem-tendril:

In Passiflora grandiflora (Fig 9.10-A) the axillary-stem is modified into tendril; in Cissus quadrangular, Vitis quadrangularis) the stem apex is modified into tendril, in Cucurbita maxima the tendril is developed in a position lateral to the leaf but it may be a displaced axillary bud.

(ii) Inflorescence tendril:

The terminal portion and upper branches of inflorescence are modified into tendrils in Antigonon leptopus (Fig 9.10-B), Cardiospermum helicacabum, etc.

(iii) Stipule tendril:

Stipules are modified into tendrils in Smilax zeylanica, S. lanceaefolia,. Heterosmilax indica, etc.

(iv) Leaf tendrils:

Entire or different parts of leaves are modified into tendrils in different plants:

(a) Entire leaf modified into tendril in Lathyrus aphaca (Fig 9.10-F);

(b) Leaflets modified into tendril in Lathyrus sativas, Pisum ativum (Fig 9.10-G) Noravalea zeylenica, Sophora interrupta, etc.

(c) Leaf-tip modified into tendril in Gloriosa superba (Fig 9.10-C), Polygonatum cirrhifolium

(d) Petiole works as tendril in Clematis buchananiana (Fig 9.9-D) ; Nepenthes khasyana (Fig 9.9-E).

4. Thalloid Plants:

Thalloid form of angiospermic stem is found in many members of Podostemaceae, e.g. Hydrobryum griffithii. Entire stem is firmly anchored on rock-surface and produces flowers and fruits on the upper surface.