CUMBERS – plants that cheat

Plants grow towards light. This is because ^hey need the energy from light to make their food. If one plant grows faster and taller than another, its leaves may overshadow those of the smaller plant. This will stop the smaller plant getting vital light. It will grow more slowly, become more shaded and possibly die from light starvation.

To grow upwards, become tall and bear many leaves, a plant must have a strong stem. It must be structurally sound and so a plant grows wider at the base as it becomes taller. It produces cells which elongate and become cylinders of wood. Cylinders are mechanically strong. They are produced in a ring around the inside of the stem giving a cylinder of wooden cylinders within the stem.

Climbing plants cheat. They avoid using their food and energy to build supportive wood. Instead, they use some of their total energy to produce structures or organs to attach themselves to another plant or surface which leads upwards. The food and energy they have saved can be used to grow faster, further and higher than neighbouring plants. In this way they stay in the light and gain even more energy.

The reduction in supportive tissue, the rapid growth rate and production of large amounts of food materials is reflected in the internal construction, or anatomy, of the stems of climbing plants. There are relatively little amounts of the woody tissues called xylem and sclerenchyma but there are large amounts of the food transport tissue, phloem. In most plants a collection of water-conducting pipes and a collection of food-conducting pipes will be grouped together as a bundle, called a vascular bundle. In many climbing plants, for example marrows and cucumbers, there is an extra collection of food pipes so that the xylem is sandwiched between two layers of phloem.

Those climbers which become woody do not have wood as a solid cylinder pierced by small rays of conducting tissue. The wood runs the length of the stem in wedge shaped layers separated from each other by equally large wedges of food-conducting tissue. This may be seen in the stems of the vine, Vitis, and the Dutchman’s pipe, Aristohchia.

The redirection of energy into the production of climbing aids has given a variety of specialized structures, organs and ways of using them. It is possible to artificially arrange climbing plants into four groups on their modes of climbing.

Penetrating climbers

Penetrating climbers are mainly found within the genus Hedera, ivy. Special roots grow out from the cablelike stem. These roots can grow into the surface of a wall or the bark of a tree, anchoring the plant to it and enabling it to grow up the surface. The connection to the original root may be severed, but the plant can survive by absorbing water and nutrients from the surface covered, though they are not in any sense parasites.

The genus Hedera, is found throughout Europe and Asia, north of the tropics and sub-tropics, and even in the Canary Islands as Hedera canariensis. This is an endemic species much favoured by gardeners for its ornamental variegated foliage.

Falling climbers

Falling climbers make no positive attempt to climb nor do they attach themselves to any other plant or structure. They grow upwards until they fall over onto vegetation which they then grow through and over. Such plants are also called scramblers, stragglers or trailing plants. Their main specialization is in the form of grappling hooks or barbs growing from their outer layer or epidermis. Examples are roses and brambles of the family Rosaceae. The ‘prickles’ on their leaves and stems are outgrowths from cells of the epidermis. When they are hook-shaped the barb points back in the direction from which the shoot has come. If the stem should slip backwards the barbs may catch in the vegetation and check the slip. Galium, or bedstraw, has a similar mechanism. Numerous small prickles cover the stem and leaves of the plant, making it a very efficient straggler.

The genus Drusa is a rather peculiar member of the family Umbelliferae. Its flowers are not arranged in an ‘umbel’ but are solitary, and it has a very odd distribution, being found in the Canary Islands and South America. It is a straggling plant with numerous fragile epidermal ‘prickles’ on its stems, leaves and fruits. These epidermal structures also secrete sticky substances which help the plant maintain contact with its support.

Twining climbers

Twining climbers curl around a support and grow up it. The mechanical strength of their spiral, and the friction of their surface contact with the support, stop them from sliding down. The mechanism by which this occurs consists of two stages.

The first stage is that before contact with a support is made. The shoot of the plant grows upwards or sideways in a helical fashion. It does not grow in a straight line, but moves round at varying angles as it grows. This may be related to the way the cells of the stem are produced spirally at its apex.

Cells are not produced all together as a single mass. Cells are formed in a spiral around the central dome of the apex. They will therefore be at different stages in their development at any one time. Once produced, their development, differentiation and growth will be under the delicate and complex control of many biochemical compounds, including plant hormones. Some of these enable the cellulose walls of young plant cells to become more ‘plastic’. Increased water pressure inside the cells causes them to elongate. This can give rapid elongation of the stem (or root). It is possible that the pattern of cell production and their subsequent differences in development at the time of elongation causes uneven elongation to occur, giving the circling motion known as circum-nutation. All plants show circum-nutation to some extent, though in many twining plants it is very marked. Hamulus, the plant which gives hops used in brewing, circum-nutates in a clockwise direction, while convolvulus, the bindweed, circum-nutates in an anti-clockwise direction. Once a support has been touched by the circum-nutating shoot, the second stage of the twining process begins.

This second stage is dependent on the stimulus of touch on the surface of the shoot. It would seem that hormonal control of growth is again involved, but in this case as a direct response to an external stimulus. The surface of the shoot furthest away from the touch stimulus grows longer more rapidly than the contact surface. This suggests that the touch stimulus has slowed down, or inhibited, hormone production at the contact surface. It causes the shoot to bend inwards towards the contact and grow around the support. Once begun this twining, or coiling, pattern of growth is self-perpetuating. Growth in response to touch is called thigmotropism.

Plants maintain the same direction during twining as they show in circum-nutation, hop and honeysuckle twining clockwise and bindweed anti-clockwise. Other twining plants are Clematis, Bryonia (bryony), Aristolochia (Dutchman’s pipe) and the popular ornamental Stephanotis fioribunda.

Grasping climbers

The pattern of growth in grasping climbers is similar to that of twining climbers. There is an initial growth phase during which circum-nutation may occur, then after contact has been made thigmotropic coiling around the support occurs. The essential difference between this group and the twiners is that the grasping plants have separate organs specialized for the grasping and coiling. These organs are known as tendrils. They are very common in the legume family Leguminosae, and the gourd family Cucurbitaceae. The pea, Pistim sativum, and the sweet pea, Lathyrus odoratus, are legumes, while bryony, Bryonia cretica, and cucumber, Cucurbits, belong to the gourd family.

Tendrils often develop instead of the end leaflet of a pinnate leaf, or in place of any or all of the leaflets. Lathyrus aphaca, the yellow vetchling, has tendrils instead of leaves. Photosynthesis is carried out by the greatly enlarged stipules.

During the circum-nutatory or ‘searching’ phase the tendrils remain mechanically weak. This keeps them flexible though fragile. Support is mainly due to water pressure inside the cells. However, once contact has been made and the tendril has coiled around the support, woody strengthening fissue may be laid down. The tendril continues to coil around the support and along its own length; so that the plant and support are pulled closer together. This coiling gives the tendril extra strength while keeping it flexible enabling it to remain intact on windy days when violent movements may occur. Not only leaflets can develop as tendrils; leaf-stalks, leaflet-stalks and elongated leaf mid-ribs may act in a tendrillike fashion. The Chinese Partheuocissus tricuspidata (Ampelop-sis vcitchii) even has tendrils which develop sticky pads at their ends. The plant does not grasp; it sticks instead.

The climbing and straggling habit is widespread throughout most of the climatic areas of the world. It is generally a weedy feature used for the exploitation of other plants and is frequent among colonizers of disturbed environments. As many of the climbers are good biological weeds, they grow rapidly and flower profusely. This has made them popular ornamental plants. Unchecked they can rapidly become unpopular horticultural weeds.

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