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The following points highlight the top three special types of special flowering plants. The types are: 1. Saprophytes 2. Parasites 3. Halophytes.
Special Flowering Plants: Type # 1. Saprophytes:
These are plants that grow in places rich in decaying organic substances, and derive their nutrition from them. Among the angiosperms Monotropa (Indian pipe) and some orchids afford good examples of saprophytes. Total saprophytes are colourless; and the partial ones are green in colour.
Their roots become associated with a filamentous mass of a fungus which takes the place of and acts as the root-hairs, absorbing food material from the decomposed organic substances present in the soil. The association of a fungus with the root of a higher plant is known as mycorrhiza.
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Mycorrhizal fungi commonly associated with the roots of many forest trees, orchid seedlings, pine seedlings, etc. Two types of mycorrhiza are seen— endotrophic, in which the fungus is internal, usually living within the cortical cells of the root, as in many orchids, and ectotrophic, in which the fungus is external, growing attached to the surface of the root, as in conifers and other plants.
Special Flowering Plants: Type # 2. Parasites:
The plants that grow upon other living plants absorb their food material from them. For the purpose of absorption parasites produce special roots, the haustoria, which penetrate into the tissue of the host plant and absorb nutrition from it. There are differential degrees of parasitism. Some are total parasites and other partial parasites.
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The total parasites are never green in colour as they obtain all their food from the host plant; while the partial parasites develop chlorophyll and are in a position to manufacture food to a greater or less extent.
They may be parasitic on the stem and branches or on roots. Accordingly they are said to be stem-parasites or root-parasites (e.g., total stem parasite—Cuscuta; partial stem parasite—Viscum; total root parasite Orobanche, and partial root parasite, Striga). (See figs. 45.2 and 45.3)
Special Flowering Plants: Type # 3. Halophytes:
These plants grow in saline soil or water, where there is abundance of salt in the soil; hence halophytes show some special characters. Most of halophytes have succulent leaves and some possess succulent stems. Leaves may be modified into or provided with spines. Typical examples of halophytes are Suaeda maritima, Salsola, Acanthus ilicifolius, Chenopodium, Basella and some species of Asclepiadaceae.
Halophytes growing in marshy places near the seashore, as in Sundarbans (West Bengal), form a special vegetation known as the mangrove. Mangrove plants produce a large number of stilt roots from the main stem and the branches.
In many cases, in addition to the stilt roots special roots, called respiratory roots or pneumatophores, are also produced in large numbers, such roots develop from underground roots, and projecting beyond the water level they look like so many conical spikes distributed all round the trunk of the tree.
They are provided with numerous pores or respiratory spaces in the upper part, through which exchange of gases for respiration takes place. Mangrove species also show a peculiar type of germination. The seed germinates inside the fruit while it is still on the parent tree and is nourished by the same. Germination is almost immediate without any period of rest.
The radicle elongates to a certain length and swells at the lower part. Ultimately the seedling separates from the parent tree and falls vertically down. The radicle presses into the soft mud, keeping the plumule and cotyledons clear above the saline water.
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This kind of germination of the seed inside the fruit while the latter still remains attached to the plant is known as vivipary. Typical examples of mangrove plants are—Rhizophora, Sonneratia, Ceriops, Heritiera Excoecaria, etc. (See figs. 45.4 to 45.10)
The mangrove plants exhibit several xeromorphic characters:
The leaves are succulent. Sometimes the stems also become succulent. Many species are covered with hairs (e.g., Avicennia). The epidermis is thick-walled and strongly cutinized. The sunken stomata are found beneath the epidermis. Usually aqueous tissue is present.
The mesophyll is almost devoid of intercellular spaces and the palisade tissue in the main chlorenchyma (e.g., Sonneratia). The nerve ends dilate into water storing tracheids (e.g., Avicennia). Long stone cells or bast like mechanical cells are lodged in between the palisade cells (e.g., Rhizophora, Sonneratia, etc.). Mucilage cells also occur (e.g., in Rhizophora, Sonneratia, etc.).
Genus Rhizophora:
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T.S. of Stem:
Epidermis:
The epidermis is composed of variously shaped cells appearing conical in transverse sections. The epidermis frequently consists of more than one layer, but a true hypodermis of 3-7 layers is also common. The cork in young stems generally arises superficially, usually in the hypodermis. The young stem has a very thick cuticle.
Cortex:
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The primary cortex is lacunar. H-shaped sclerenchymatous idioblasts are present. The cells of cortex possess pitted walls and are full of tannin and oil. Calcium oxalate crystals are also present. The inner cortex has groups of branched sclereids which give mechanical strength to the lacunate cortex.
The sclereids are lignified thick walled cells with narrow lumina. The endodermis is conspicuous. The endodermal cells possess starch grains.
Pericycle:
It consists of a sub-continuous composite ring of sclerenchyma consisting of 3-4 layers of cells.
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Vascular Bundles:
The vascular bundles are conjoint, collateral, endarch and open.
Xylem:
The xylem is traversed by rays 2-3 cells wide in Rhizophora mucronata. The vessels possess scalariform perforation plates.
Crystals:
The crystals are generally clustered.
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Secretory Elements:
Vertically elongated secretory cells containing tannin and/or oil present in the cortex and the pith, (see fig. 45.4).
T.S. of Petiole:
Transverse sections of petiole exhibit a ring of bundles surrounding additional medullary strands in Rhizophora mucronata Lam. Mostly clustered crystals present. Secretory elements- cells containing tannin have been recorded in the mesophyll, cortex and pith. The inner cortex possesses H-shaped idioblasts.
Anatomy of Leaf:
The leaves are usually dorsiventral. Hairs are mostly unicellular with thick or thin walls. The cuticle is well-developed and often quite thick on both the leaf surfaces. Cork warts occur as small black spots on the lower side of the leaf. The epidermis is single layered and consists of rectangular cells.
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The cells of upper epidermis possess some rod-shaped and cubical crystals of calcium oxalate. The hypodermis towards the upper surface is 2 or more layered. The stomata are confined to the lower surface. They are depressed and often provided with a front cavity. The mesophyll consists of palisade and spongy tissue.
Palisade tissue consists of 1-4 layers. Spongy tissue usually possess large intercellular spaces. Aqueous tissue and mucilage cells are also present beneath the upper epidermis. H-shaped sclerenchymatous idioblasts occur in the palisade tissue, and variously branched ones in the spongy mesophyll.
Anatomy of Breathing Root:
The well-known breathing roots (pneumatophores) of members of the Rhizophoraceae inhabiting mangrove swamps have large intercellular spaces of the spongy cortex which facilitate gaseous exchange in the special habitat in which the plants grow. The cork consists of suberized cells.
The cork layer remains interrupted by lenticels at several places. In some cases the cork consists of alternating layers of suberized and ordinary parenchymatous cells in the aerial portion. Next to the cork there lies a thick cortex containing abundant, large, intercellular spaces, arranged radially around the stele.
Intercellular spaces are larger in the subterranean than in the aerial parts of the roots. The cortex may be subdivided into two portions-the secondary cortex and the primary cortex. The secondary cortex consisting of few layers is found just beneath the cork. The primary cortex lies next to secondary cortex. The primary cortex is broad and lacunar.
The primary cortex in the terrestrial part of the root is composed of cells of two kinds:
(a) Radially elongated cells connected with one another tangentially by short lateral arms;
(b) Rows of vertically elongated cells which show small, circular lumina in transverse sections. Solereder has recorded the ridges of thickening which are supposed to give mechanical support to the radially elongated cells where they are found.
However, according to Bowman these are really cells filled up with mucilaginous sap. Mullan (1933) states that the cortex in the aerial part of the root system of Rhizophora mucronata Lam. is much reduced and the lacunae small. Sclerenchymatous idioblasts also occur in the cortex.
H-shaped sclerenchymatous idioblasts project into the cortical intercellular spaces in the aerial part of the root, but are less numerous below ground. Mullan (1933) has also recorded the vertically elongated tubular cells, filled with tannin and oil, situated at the junctions between the branched cells in the terrestrial part or the root of Rhizophora mucronata.
The endodermis is conspicuous. Numerous secretory cells are found in the peri-cyclic region. The xylem is well formed and strongly lignified. The outer pith is sclerenchymatous whereas the central region of pith is parenchymatous. Several oil cells are present here and there in the pith region.
Genus Sonneratia:
The members of family Sonneratiaceae are generally secretory cells trees. Some of the species of Sonneratia inhabiting Mangrove Swamps from Africa to Australia are provided with vertical branches of the root system projecting into the air or water above the mud. The occurrence of intraxylary phloem in Sonneratia is also noteworthy.
T.S. of Stem:
The young stem of Sonneratia is provided with collenchymatous wings. The cork arises in the sub-epidermis in Sonneratia apetala Ham. Primary Cortex consists of spongy parenchyma. The outer part of cortex contains branched sclerenchymatous idioblasts. Xylem is found in the form of a continuous cylinder traversed by narrow rays.
The vessels possess simple perforations. The phoem includes sclerenchyma. The endodermis is inconspicuous. The pericycle is found in the form of patches over phloem. The intraxylary or inner phloem is found next to xylem around the pith. The pith remains supported by sclerenchymatous elements.
The cells of the pith and cortex contain tannin and oil drops. The secretory cells occur in the phloem as well as in intraxylary phloem, of Sonneratia apetala. According to Mullan (1933), crystals in vertical rows of special cells in the secondary phloem of Sonneratia apetala.
The secondary cortex is composed of tangentially elongated cells. Sclereids are also found in the secondary cortex. These cells contain oil drops and tannin.
Anatomy of Leaf:
The leaves of Sonneratia apetala Ham are isobilateral. The upper epidermis possesses cuticular ridges. Cork warts are found in some species of Sonneratia. The stomata are deeply sunken and present on both surfaces in Sonneratia acida Linn. The stomata are of rubiaceous type and they are equally numerous on both surfaces in Sonneratia apetala Ham.
Large and rounded mucilage cells are present below the upper epidermis in the mesophyll. The central region consists of aqueous tissue and contains branched sclerenchymatous idioblasts. The oil and tannin cells are also present in the mesophyll tissue. (see fig. 45.8)
Anatomy of Breathing Root:
The vertical, negatively geotropic portions of the root system are known as ‘breathing’ or ‘pneumatophores’. The terrestrial roots are also present.
The lenticels are present on the pneumatophores. The cork consists of a succession of lamellae each composed of three layers of cells. The cells of the outer layer of the cork are rounded externally and not suberized. The cells of the middle layer of the cork are tabular and suberized and those of inner layer are radially elongated, rounded on the inner side and suberized.
Two layers of rounded cells, present between each succeeding cork lamella, facilitate separation at these points. The cortex is composed of rounded cells interspersed with schizogenous, intercellular spaces. The cortex also contains cells with crystals and sclerenchymatous idioblasts.
The cortex at the proximal end of the pneumatophore is supported by curved, lignified spicules. The phloem includes isolated strands of fibre and vertical rows of crystalliferous cells. The xylem is found in the form of a broad feebly lignified ring. The pith contains sclerenchymatous idioblasts at the distal end but not at the proximal end.
The teirestrial roots of Sonneratia show the following characteristics.
Cortex is lacunar. It is composed of cells mostly triradiate, and supported by thickening ridges and by vertically elongated cells with thick, pitted but lignified walls. Cork arises superficially.
