The early growth stage of the plant, juvenile growth, is characterized by certain physical appearances and activities that are different from those found in the later stages or in adult growth. Often leaf shapes vary; e.g. the juvenile ivy leaf is three-lobed while the adult leaf is more oval, as shown in Figure 11.4. The habit of the plant is also different; the juvenile stem of ivy tends to grow horizontally and is vegetative in nature, while the adult growth is vertical and bears flowers. Other examples are common in conifer species where the complete appearance of the plant is altered by the change in leaf form, for example, Chamaecyparis fletcheri and many Juniperus species such as J. chinensis. In the genera Chamaecyparis and Thuja, the juvenile condition can be achieved permanently by repeated vegetative propagation producing plants called retinospores, which are used as decorative features.
Leaf retention is also a characteristic of juvenility. It can be significant in species such as beech (see Figure 11.5), where the phenomenon is exaggerated, and the trees
The juvenile stage is a period after germination that is capable of rapid vegetative growth and is unlikely to flower.
can be pruned back to the vegetative growth. This can create additional protection in windbreaks although the barrier created tends to be too solid to provide the ideal wind protection (see p38).
Many species that require an environmental change to stimulate flower initiation, such as the Brassicas that require a cold period, will not respond to the stimulus until the juvenile period is over; about eleven weeks in Brussels sprouts.
The adult stage essential for sexual reproduction is less useful for vegetative propagation than the responsive juvenile growth, a condition due probably to the hormonal balance in the tissues. Figure 11.4 shows the spontaneous production of adventitious roots on the ivy stem. Adult growth should be removed from stock plants (see p174) to leave the more successful juvenile growth for cutting.
The ability of the plant to reproduce vegetatively is widely used in horticulture and these methods, both natural and artificial, are detailed in Chapter 12.
Although the life cycle of most plants leads to sexual reproduction, all plants have the potential to reproduce asexually or by vegetative propagation, when pieces of the parent plant are removed and develop into a wholly independent plant. All living cells contain a nucleus with a complete set of genetical information (see genetic code, Chapter 10), with the potential to become any specialized cell type. Only part of the total information is brought into operation at any one time and position in the plant.
If parts of the plant are removed, then cells lose their orientation in the whole plant and are able to produce organs in positions not found in the usual organization. These are described as adventitious and can, for example, be roots on a stem cutting, buds on a piece of root, or roots and buds on a piece of leaf used for vegetative propagation. Many plant species use the ability for vegetative propagation in their normal pattern of development, in order to increase the number of individuals of the species in the population. The production of these vegetative propagules, as with the production of seed, is often the means by which the plant survives adverse conditions (see overwintering), acting as a food store which will provide for the renewed growth when it begins. The stored energy in the swollen tap roots of dock and dandelion enable these plants to compete more effectively with seedlings of other weed and crop species, which would also apply to roots of Gypsophila paniculata, carrots and beetroot.
Stems are telescoped in the form of a corm in freesia and cyclamen, or swollen into a tuber in potato, or a horizontally growing underground rhizome in iris and couch grass. Leaves expanded with food may
developing leaf developing flower swollen base of stem (new corm)
-old corm fibrous roots
swollen leaves developing flower axillary bud (next year's bulb)
stem adventitious root
Figure 11.6 Structure of organs responsible for over-wintering and vegetative propagation adventitious roots
Figure 11.6 Structure of organs responsible for over-wintering and vegetative propagation form a large bud or offset found in lilies. A bulb, as seen in daffodils, tulips, and onions, is largely composed of succulent white leaves enveloping the much reduced stem, found at the base of the bulb (see Figure 11.6).
Other natural means of propagation include lateral stems, which grow horizontally on the soil surface to produce nodal, adventitious roots and subsequently plantlets, e.g. runners or stolons of strawberries and yarrow. The adventitious nature of stems is exploited when they are deliberately bent to touch the ground, or enclosed in compost, in the method known as layering, used in carnations, some apple rootstocks, many deciduous shrubs such as Forsythia, and pot plants such as Ficus and Dieffenbachia. The roots of species, especially in the Rosaceae family, are able to produce underground adventitious buds that grow into aerial stems or suckers, e.g. pears, raspberries. By all these methods of runners, layering and suckers, the newly developing plant (propagule) will subsequently become detached from the parent plant by the disintegration of the connecting stem or root.
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