Category Science

     In plants which show secondary growth the outer appearance of the stem differ in different species f plants. This difference results from the manner of growth of the periderm, the structure of the phellum and the nature and amount of tissue that are separated by the periderm from the stem.

            The periderm consists of three parts: the phellogen which is the cork cambium, the phellum which is the cork produced centrifugally by the phellogen and phelloderm which is parenchymatous tissues produced centripetally by the phellogen.

            In trees which produce successive periderms by the formation of successive phellogens up to the depth of phloem, there will be many cork layers.

            All the cork layers together with cortical and phloem tissues external to the innermost phellogen are termed rhytidome. In such trees the colour of the stem will be dark brown and never green.

            In plants like citrus, eucalyptus, acer and acacia the development of periderm commences only after the production of the secondary vascular tissue has reached considerable dimensions. In such cases the circumference of the epidermis increases together with secondary and other tissues on the outer side of the cambium.

            In viscum cork tissue is never formed and the epidermis increases in circumference and persists on the stem throughout the life of the plant. In all these plants stem surface looks green even after secondary growth.

            In plants like solanum, guava, pyrus and nerium the first phellogen is formed in the epidermis itself and iln plant like populous, jugulans and ulmus the first periderm is formed in the outer most cortical layer next to the epidermis.

            In such cases the subsequent periderms are not formed to the full circumference of the stem similar to the formed one. But they develop in the form of scales. So in these plants absence of well marked rhytidome give the stem a green appearance even after the secondary growth.

            The timing of leaf loss varies with species, site and season. Day length and temperature are the two triggers for colour change and leaf loss.

            The timing is usually species-specific but is also related to site conditions. For example, a fairly dry season would result in some trees leaves dropping early, before they had turned, in a reaction to the drought stress; leaves may also die on the tree but hang on until much later. Species variations are also important. Norway maples normally have green and fully functional leaves that keep on photosynthesizing until two or three weeks after leaves of sugar maples have turned. If both are on a cramped site, Norway maples, with extra weeks of energy storage, may outgrow and outlive sugar maples.

            Oaks keep their leaves much longer than many other species because a layer of cells that forms where the leaf stem is attached, called the abcision layer, does not form a complete barrier. In the beech trees, which are in the same family, an incomplete layer is seen in younger trees, but mature beeches, 25 to 30 years old, form a complete layer. There are also sex differences; leaves of female ginkgo trees usually colour and drop earlier than those of males. And trees near street lights may be affected by the longer light exposure and keep their leaves longer.

    The red portion in the stem of cane is due to a fungal disease called red-rot caused by the organism Glomerella tucumanensis. The organism attacks during the conidial stage (imperfect stage) when it is known as Colletotrichum falcatum.

            The pathogen infects the host mainly through the leaf scars at abscission or immediately thereafter, enters the parenchyma, grows intracellularly in the early stages, and forms an intercellular mycelium in the later stages. The fungal hyphae penetrate the host’s cell wall during the progressive stage of the disease forming minute penetration pegs. These pegs expand to the normal hyphal diameter immediately after reaching the other side of the cell wall. This mechanical pressure causes the dissolution of the tissue. Thus the tissue dissolution is not due to enzyme action, but due to mechanical pressure.

            But hydrolyzing enzymes are produced at a later stage when the tissues begin to die and the pathogen grows on the dead cells of the host, that is, in the saprophytic phase of the fungus. Only at this time reddening of the stem vascular tissue occurs followed by the formation of lysigenic cavities. At this stage when the affected canes are split open, the tissues of the internodes which are normally white or yellow-white will become red in one or more internodes usually near the base.

            The reddening is conspicuous in the vascular bundles and progresses towards the pith. When such diseased shoots appear in the field, secondary infection is caused by conidia which are produced in aierouli (asexual reproductive bodies) and transmitted through insects, wind and water.

 

         

 

 

 

 

 

  The bipinnate compound leaves of Mimosa pudica, touch-me-not plant, have a swollen base called pulvinus which has two distinct halves. The lower half below the vasular strand is made of thin walled parenchyma cells with larger intercellur spaces and the upper half has slightly thick walled parenchyma cells with a few small intercellur spaces.

            Under normal conditions, the cells of both the halves remain turgid. When the touch stimulus reaches the pulvinus the osmotic pressure in the lower half of pulvinus falls. As a result they release water into the intercellur space and become flaccid. But the upper half maintains turgidity the pressure excerted by which causes the leaves to drop down.

            The leaflets also have similar swollen bases but are smaller and are called pulvimules. The touch stimulus is first perceived by these pulvimules. Here also the process occurs which results in the folding of the leaflets. When the stimulus is passed on to the stalk base the entire leaf droops down.

            The touch-me-not plant shrinks within a few minutes of being touched. This is due to the loss of turgidity by cells within the pulvini-specialized motor organs at leaf joints. Upon stimulation the leaf cells lose a potassium ion which causes water to leave the cells by osmosis. It takes about 1 o minutes for the cells to regain turgidity and the leaflets to open out.

  Crotons are ornamental plants grown for their variegated leaves. The different coloured patches in these leaves are due to the presence of chromoplasts in the leaf cells. Chromoplasts contain coloured pigments, other than chlorophyll, which can reflect or transmit light, or both.

            The colour of a pigment depends on its selective absorption of certain wavelengths of light and its reflection of others. Carotenoids are a group of red, orange, and yellow pigments and contain many catalytic members. Some carotenoids act as accessory pigments in photosynthesis, transferring the light energy they absorb to chlorophyll for conversion to chemical energy.

            Chemically, pigments fall into a number of minor groups, arbitrarily divided into 2 major groups. The first group comprises pigments that contain nitrogen; it includes chlorophyll and dark coloured pigments called melanin.

            Related to melanins are the indigoids, of which the well known plant pigment indigo is an example. Riboflavin, also known as vitamin B12, is one of a number of pale yellow to green pigments produced by several plant groups.

            The second group is formed of pigments without nitrogen. Carotenoids are members of this group, as are the important plant pigments called flavonoids. In leave, flavonoids selectively admit light wavelengths that are important to photosynthesis, while blocking out UV light, which is destructive to cell nuclei and proteins.

            Bright colours are produced by the conversion of colour less flavonoids, called flavonols, into coloured forms, called anthocyanins. Quinones provide many yellow, red and orange pigments.

            Venus fly-trap, an insectivorous plant, normally grows in swamps and moist soils characterized by lack of sufficient nitrogen (as nitrates). Their root system is also not so well developed. As a result these plants tend to trap insects and ‘digest’ them to augment their nitrogen supply. These carnivorous plants do not have any special mechanisms or honey secretions to attract insects but only modified leaf traps (Dionaea muscipula), vase-like leaves (Nepenthes Khasiana), leaf hairs having glue on their tips (Drosera) and leaf surface having a sticky coating (Pinguicola alpina) to trap them. In Venus fly-trap plant, the two halves of the leaf blades can swing upward and inward as though hinged.

            Inside the hinged portion of each leaf are several long trigger hairs. As the insect walks along the leaf surface and touches these hairs, it stimulates a hydraulic response in the leaf-cells and makes them lose water rapidly. This causes the leaves to close. Long projections along the leaf margins help in trapping the insect.

            Once an insect is trapped, digestive enzymes are secreted by the hairs which ingest the insect and absorb the contents. After a meal, the trap opens again only after several days. Generally each modified leaf is used to trap only 3-4 insects before it falls.

            These plants also have chlorophyll by which they can photosynthesis to cater to their energy (food) requirements. Hence these plants are not obligatory carnivorous forms. But they can grow exuberantly to produce flowers and seeds, if insects are available, as they supplement their nitrogen supply.