External Morphology of Higher Plants (With Diagram)

The below mentioned article provides an overview on the external morphology of higher plants with diagram.

A critical study of the external morphology or the external forms of higher plants is necessary primarily in order to be able to describe the plants accurately and to enable one to distinguish between similar-looking plants.

This is a point essential in the study of Systematic Botany as well as in studying variations in connection with Evolution and Genetics.

Out of the general science of Morphology other sciences like Experimental Morphology, Morphogenesis, etc., have sprung up which try to find out why and how each organ grows.

The developmental study of an organ gives a true insight into the nature of an organ. Embryology (which is more internal than external morphology) or the study of the development of embryos has developed so much that it is sometimes treated as a separate science.

While speaking about the general morphology of Angiosperms one means the mor­phology of typical forms, that is, the ordinary plants we see around us like the mango tree, the rose shrub or the grasses. Such plants grow on soil (terrestrial), live in a moderate environment and are completely independent for their food supply (autotrophic}. But one should have some idea of the exceptional forms one usually meets.

According to the mode of living, plants may be classified as follows:

I. Autotrophic Plants or Autophytes:

These are the normal plants living inde­pendently, building their own food out of inorganic raw material. There may be diff­erent types of them according to their ecological environments. Different environments cause differences in their morphology. Thus, we find special adaptations in aquatic plants, terrestrial plants, xerophytes, mangrove plants, etc.

Among them may be considered the Epiphytes as, ad though they are peculiar, they are completely autotrophic. These plants usually grow on the top of other plants but are not parasitic on them. They have two types of roots.

Some are clinging roots which are rather small and creep into the cracks in the bark of the tree on which the epiphyte grows. These clinging roots not only fir the epiphyte to its support but also absorb the little nutriment that is found within the debris accumulating on the bark.

The other roots are the aerial roots which are more pro­minent and hang about in the Sir. These aerial roots are usually green and covered by a special fibrous tissue called velamen which is sponge-like in absorbing the moisture con­tained in air as well as rain water. Most orchids are epiphytes. There are also instances of epiphytes among some Pteridophytes like ferns. Ficus tree like banyan or peepul often begin their lives as epiphytes.

Some seed may be deposited (say, with the excreta of a bird) on the top of a date palm or on the roof of a house where the seedling grows as an epiphyte. But, later on, these epiphytic Ficus plants send their roots down to the soil. Once the plant gets a foothold on the soil it becomes strong and gradually smothers its support or destroys the building. Quite reverse is the case of Scindapsus offi­cinalis (gajapipul) of Araceae  which begins its life as a terrestrial plant but, after climbing up, gets detached from the soil and becomes an epiphyte.

II. Heterotrophic Plants Obtain their Nutrition by Irregular Methods:

The nut­rients are of organic origin and, therefore, derived from living or dead organisms. Heterotrophic plants may entirely depend on this irregular nutrition or substitute it by regular inorganic nutrients. These are of different types according to the special modes of nutrition. When an organism takes its position on another living organism and sucks the nutriments prepared by the latter for its own use, it is called a parasite.

The second plant or animal, at whose expense the parasite is growing, is called the host. If the depen­dent organism lives, not on a living host, but on its dead body or on its decayed pro­ducts (e.g., rotten, leaves, humus or dung) then it is called a saprophyte. It is not always possible to demarcate between parasites and saprophytes as a parasite growing on a living host may continue to grow after the host is dead.

Sometimes, two plants may live together in organic association without causing harm to either of them. The plant A may prepare one type of nutrition (say, nitrogenous) and pass a part of it to the plant B while the plant B may produce another type of nutri­ments (say, carbohydrates) and pass some of it to A. This is a case of mutual helping and not parasitism and is known as symbiosis.

Of special interest are the Insectivorous or Carnivorous plants which get-their nitrogenous nutrients from small animals like insects. This is a case of feeding on animal protein and so does not come under parasitism.

A. Parasites:

A parasite may depend for its nutrition on its host totally or partially. When par­tially dependent, it retains its green colour with which it prepares its own carbohydrates. These are called partial parasites or semi-parasites.

When the parasite obtains all its food supply from its host it does not even possess chlorophyll and is called a total parasite. Some angiospermic parasites remain confined to the root or to’ the shoot. Accordingly, they are named as root parasites or stem parasites.

1. Partial Parasites:

Among partial stem parasites the Loranthaceae is an important family. The mistletoe and Loranthus sp., specially the latter genus, are quite common on large trees like mango and often cause serious damage.

They possess thick green leaves and remain firmly perched on branches by means of haustoria. The seeds of these plants are sticky and, when left by birds on the twigs of plants, germinate there with the radicle coming out as a sucker. Germination does not proceed far if a suitable host be not obtained. The sucker fixes the plant on the host. In Viscum this sucker, after penetrating the host tissues, gives out branches which form an internal sucking system.

The Loranthus sucker, on the other hand, grows externally along the bark and gives out haustorial discs here and there. The haustorial discs or suckers connect the vascular systems of the two plants so that the xylem stream from the host is drawn into the para­site. Cassytha filiformis  of Lauraceae is another partial stem parasite found in the tropics.

The plant is a leafless wiry stem devoid of normal roots which entwines the host plant and comes in contact with the conducting tissues of the host by means of haustorical discs here and there.

Cassytha is a partial parasite because the stem is greenish and is capable of some carbon assimilation.

Partial root parasites are not as common as the stem parasites. The sandal wood tree (Santalum album of Santalaceae), grown in a commercial scale in Mysore, develops as a root parasite.

Some of its roots penetrate their sucking roots into the roots of nearby plants. Although it possesses independent roots as well, it does not grow luxuriantly if the host plants are not grown side by side.

In cooler climates there are several plants which obtain their nutrition through roots attached to grasses and other plants. Thesium and Rhinanthus are such examples. They often seriously damage pastures.

2. Total Parasites:

Total parasites are completely dependent on their hosts and are devoid of func­tional roots.

Among total stem parasites the golden yellow coloured twiner dodder (Cuscuta reflexa of Convolvulaceae) is very common. The plant is formed solely of the twining wiry stem devoid of leaves.

Even the embryo is devoid of cotyledons. The seed germinates as a terrestrial plant but, as soon as it gets its host, it forms a dense entanglement on it and loses all connec­tion with the soil. There are haustorial discs at Short intervals fixing the parasite on the stem of the host. The haustoria penetrate the conducting strands (both xylem and phloem) and, sometimes even the pith of the host.

Although the foliage leaves are suppressed, there is no reduc­tion in the flowers which arise in clusters from the nodes in the flowering time and there may even be small non-green bracts subtending the flowers. A very interesting total stem parasite is Arceuthobium minutissimum (Loranthaceae) growing on Pinus excelsa on the Himalayas.

This is reported to be the smallest dicotyledon with no stem but an inconspicuous mycelium-like stock ramifying within the bark. The minute flowers burst out of this bark here and there.

There are quite a few interesting examples of total root parasites. Orobanche sp.  of Orobanchaceae is a leafless parasite growing on the roots of a number of horticultural crops like brinjal, tobacco, tomato, potato, mustard, etc. The seed germinates only on contact with root of the host and forms an underground tuber. Then it gives rise to an aerial inflores­cence of coloured flowers with brown bracts.

Another plant of similar habits is Aeginetia acaulis, also belonging to Orobanchaceae, which is common in the Orissa forests being a parasite on the roots of khus-khus (Vetiveria zizanioides) and weeds in the shade of shal (Shorea robusta) forests.

Quite a similar example is Balanophora dioica of Balanophoraceae found on tree roots at an elevation of about 6000 ft. in Assam. This also is represented by leafless inflorescences.

The most interesting example among total root parasites is that of Rafflesia arnoldi  found in the East Indies. Parasi­tism has developed to the extreme here. The plant body is a tissue forming a branched filamentous body ramifying inside the tissue of the roots (sometimes stem also) of its host (Cissus, Vitis, etc.).

The vegetative body has, therefore, been reduced almost to the level of the fungi. But, when flowering time comes, large buds begin to develop on this tissue and ultimately burst out of the roots. The buds open out to form some of the largest flowers known, sometimes reaching one metre in diameter and 15 Kg in weight.

The flower has a fleshy colour and the smell of rotten meat which attracts carrion flies for pollination. Sapria himalayana, a relative of Rafflesia and of similar habits grows in the extreme Eastern Himalaya (Abor Hills).

B. Saprophytes:

Saprophytes live on dead organisms or decayed organic products. They are quite common among fungi and bacteria.

Among the higher plants there are some orchids (Corallorhiza, Neottia, Epipogium, etc.) and a few other plants like Monotropa, Sarcodes, etc., of Pyrolaceae which are total saprophytes in the sense that they are not even green and they obtain all their nutrition from decayed vegetable matter (humus) on the forest floor.

There are also partial saprophytes which are green plants obtaining part of their nutrition from organic substances. But, these higher plants are not found to utilise the organic nutrition directly. This is done through some fungi living in their roots. These plants are therefore better considered in connection with Mycorrhiza.

C. Symbiosis and Mycorrhiza

The cooperative growth of algae and fungi to form lichens is well known. Some Angiospermic plants are also seen to grow in conjunction with fungi. All of them come under symbiosis (living together). A very common type is known as mycorrhiza, where a fungus is found in the root of a higher plant. The amount of help that the higher plant receives from the fungus is not always clear.

Two types of such mycorrhiza are re­cognized—ectotrophic (when the fungus lives outside the root tissue) and endotrophic (when the fungus grows inside the root cortex). Beside these, there is the special type of symbiosis shown by leguminous root nodules. Mycorrhizal plants often fail to grow without a proper infection by the specific fungus.

1. Ectotrophic Mycorrhiza:

Fungi are known to occur regularly forming a mantle outside (hence, ectotrophic) the roots of trees of Fagaceae (oak, beech, etc.), Betulaceae (birch) and some Coniferae (pines ). The hyphi penetrate only between the outer cells and replace the normal root-hairs.

In extreme cases we get the total saprophytes, where the roots are comple­tely covered by the mantle of fungal hyphi so that the plant has to obtain all its nutrition through these fungi and do not even bear any chlorophyll.

Such total saprophytic mycorrhiza is seen in Monotropa (the Indian pipe), Sarcories (Snow plant), etc.

2. Endotrophic Mycorrhiza:

In these cases the fungi occur internal to the root tissue. In the case of ordinary green orchids the seed does not germinate properly if it does not get infected with some specific fungus at an early stage.

Possibly, the young, seedling gets some of its nutrition through this fungus. But, even in these cases, the fungus becomes unnecessary when the plant matures.

On the other hand, there are certain non-green orchids (Corallorhiza, Neottia, Epipogium, etc.), which are total saprophytes. Corallorhiza rhizome branches freely under the rich humus in Pine forests and bears no root. A flowering scape bears colourless scale leaves. The rhizome is heavily infected with fungus.

Heather, ling (Calluna) and other members of Ericaceae also get infected with some mycorrhizal fungus at the seed­ling Stage. Occurrence of such mycorrhizal fungus is widespread among the Pteridophytes.

3. Plants with Root-Nodules:

Leguminous plants are well known for the presence of nodules on roots  which harbour some bacterium (e.g., Bacillus radicicola or Pseudomonas radicicola) which is able to fix atmospheric nitrogen and supply it to the host plant in the form of amino compounds.

This fact is of great importance in agriculture and in the maintenance of the nitrogen cycle. These bacteria are usually present in the soil and when the leguminous plants are
grown, the bacteria enter the roots through the root hairs. Thereafter, their presence causes the development of the nodules or tubercles as pathological growth. The nodule cells are full of the bacteria.

If, accidentally, the soil is sterile, the leguminous plants do not develop well and when grow­ing leguminous crops on such soil, inoculation with the necessary strain of bacterium may be neces­sary. It is also found that the nodules do not develop well if the soil be too rich in nitrates.

This is a true case of symbiosis as the bacteria do not harm the leguminous plants but supply them with nitrogenous substances while they receive other forms of nutri­ents in return.

D. Insectivorous Plants:

The insectivorous (or car­nivorous) plants have attracted the imagination of literary men ever since they were known. One may read stories of hunting plants which prey upon large animals like dogs and even human beings. Such stories are, of course, purely imaginary. The insectivorous plants have mechanisms to trap animals no bigger than butterflies or grasshoppers and are adapted to digest the protein matter contained in these animals.

These plants depend only partially on this irregular nutrition as they possess normal leaves and roots. There is no such activity in these plants that may be compared with the beasts of prey.

Following are the types of insectivorous plants that are commonly known in India and outside:

i. Family Droseraceae:

There are a number of well-known plants belonging to this family:

a. Drosera or Sundew :

Some four species of Drosera are well known. Of these Drosera indica, D. burmanni and D. peltata Occur in India while D. rotundifolia is found in British peats and bogs where absorption by roots is difficult.

The species vary mainly in the shape of leaves which is narrow and long in D. indica, spathulate in D. burmanni, peltate in D. peltata and round in D. rotundifolia.

All the species are small herbs a few inches high. D. burmanni is seen all over Eastern India (often in rice fields) as small plants with rosettes of radical leaves which look like small reddish patches from a distance. D. peltata from Khasia Hills, the Western Himalayas and the Ghats has sometimes both radical and cauline leaves or only cauline leaves.

In all Droseras the peculiarity is that the upper surface of each leaf blade is covered with glandular hairs called tentacles which secrete a sticky fluid. This drop of fluid shines in the sun like dew drops hence the name ‘Sundew’. Insects are attracted by the glistening drops and as one alights on the leaf surface it gets entangled in the sticky fluid and all the tentacles, which are protein sensitive, bend towards it so that the insect is trapped.

It is soon killed while the glands secrete an enzyme for digesting the protein substance and the tentacles remain in a curved position until digestion is complete.

The movement of the tentacles is caused by chemical stimulus as it is seen that if a small bit of meat is placed on the leaf, the tentacles react in the same way and such movement cannot be induced by any other means. The digested protein is absorbed by the leaves.

b. Dionoea or Venus Fly-trap:

Dionoea muscipula is a well-known insecti­vorous plant from the North American bogs. The radical leaves occur in a rosette and act like rat traps. The petiole of the simple leaf is winged and the lamina is deeply notch­ed at the apex. The leaf margin is slightly incurved and provided with long pointed teeth. The two halves of the leaf are capable of movement with the midrib acting as a hinge. When the leaf folds, the marginal teeth closely fit into one another like the interlocking device of a rat trap.

The halves of the leaf normally remain slightly inclined forming an angle at the midrib. On the upper surface of the leaf are six. (three on each half) extre­mely sensitive trigger hairs and a number of reddish glands. As a trigger hair is touched by an insect the leaf closes like a trap and the glands secrete copiously a digestive enzyme which digests the protein matter. The leaf reopens only after digestion is complete.

c. Aldrovanda :

Aldrovanda vesiculosa, sometimes called Malacca jhangi, used to be found as a free-floating aquatic weed in the salt marshes to the south of Calcutta.

The plant is rootless and the slender wiry stem bearing whorls of leaves multiply vegetatively. The leaves are like miniature specimens of Dionoea. The petiole is slightly wing­ed and terminates in bristles.

The simple, roundish lamina is knotched at the top. The margin of the leaf is slightly curved inwards and bears minute teeth. The two halves of lamina are slightly inclined inwards forming an angle at the midrib.

On the upper sur­face of the leaf, specially close to the midrib, there are a number (more than six) of sensi­tive trigger hairs besides the digestive glands -which are distributed all over the surface. As soon as very small animals touch the hairs, the two halves close about the midrib effecti­vely trapping the victims inside forming something like a temporary Stomach. The diges­tive glands then secrete the necessary enzyme and the leaf remains closed until digestion and absorption of the juice is complete.

d. Pinguicula or Butterwort:

Different species of Pinguicula grow in the European bogs. One species, Pinguicula alpina, is found in the Alpine Himalayas (11,000 to 13,000 ft.). These are small herbs like Drosera with scanty development of root.

The leaves similarly occur in a rosette but are sessile and do not bear tentacles. On the sur­face of the leaf are short, stalked hairs secreting a sticky substance arid sessile glandular hairs which secrete an enzyme.

When an insect alights or crawls on the leaf surface it gets stuck up in the sticky substance and the presence of the nitrogenous substance causes the margins of the leaf to roll upwards and inwards thus trapping the victims. Secretion and digestion follows. The leaf unrolls after digestion and absorption is complete.

ii. Family Utriculariaceae:

This is another family of insectivorous plants of which Utricularia or Bladderwort  is very common. The genus is widely distributed in tropical and temperate regions with more than 200 species. About 20 species are found in ponds, ditches and marshes or growing among wet mosses in India. The commoner species are free floating, aquatic, rootless weeds just like Aldrovanda and are found in, similar surroundings.

Such a plant shows a slender elongated stem multiplying vegetatively which boars finely dissected submerged leaves looking like green roots. Numerous segments of the leaf are modified into bladders or utricles. The bladders vary in size, some larger ones being as much as one-eighth of an inch in diameter. The inner wall of the bladder is studded with 3 or 4-pionged digestive glands.

There is an opening of the bladder which is provided with a valve door which can be pushed in but cannot be pushed out. There are long erect branched hairs or bristles on the valve door as well as around the opening.

The bladders are normally filled in with water which is absorbed by the wall leaving it empty. If minute water animals (which can be barely seen with the naked eyes) now push the bristles, the valve door opens and some water carrying the organisms is slicked in.

The pressure of this inside water prevents the opening of the valve door. Death, decay and digestion follows and the-bladder is again ready for its fresh prey only after the liquid is absorbed. Sometimes hard and indigestible remnants of such organisms are found within the bladders.

iii. Pitcher Plants:

A number of plants of a few allied families (Nepenthaceae, Sarraceniaceae, etc.) show peculiar pitcher-like modifications of leaves which act as insect-catching traps.

a. Nepenthes :

This is a South Asiatic epiphytic genus of many species of which one is found in Ceylon and one in Assam. By the name pitcher plant usually this genus is signified. The lower part of the petiole is winged and the upper forms a tendrillar structure which may coil round a support to bear the weight of the pitcher-like structure above (which may be modified leaf lamina) with an immovable lid.

The lid may be coloured to attract insects and keeps the pitcher closed during immaturity. There is some dispute as to the real morphology of the different parts of the pitcher. Some hold that the winged structure is the Ieafbase, some others consider the winged part, the tendrillar part and the pitcher as different regions of the petiole. The lid is considered as either the whole lamina or the apex of it.

One thing is certain -—the pitcher is a part of the leaf, either the petiole or the lamina. The pitcher is 1 to 10 or mere inches long. There is no active mechanism for catching insects. The inner walls of a pitcher are studded with button-shaped glands which secrete a fluid partly filling up the vessel. The edge of the pitcher is so slippery and curved in such a manner that in­serts trying to have a perch on it slip down and cannot climb up the steep wall.

There are also some downwards pointed hairs below the rim which prevent the moving up of “the insects. These get drowned and are digested by the enzyme secreted from glands.

Gradually, the pitcher gets filled up with horny remnants of ants, grasshoppers, cockroaches, etc. The wrapper shows a coloured photograph of the pitcher plant as it grows in its natural surroundings in the Garo Hills in Eastern India.

b. Sarracenia :

Found in the North American bogs, these pitcher plants have pitchers forming a rosette. The pitchers are similar to those of Nepenthes but sessile.

There are a few other types of pitchers like Cephalotus, Heliamphora, etc. All such pitchers are modifications of leaves and act as insect traps in the same way.