Unitary and modular organisms 

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Unitary and modular organisms

In unitary organisms, form is highly determinate. Barring aberrations, all dogs have four legs, all locusts have six legs, all fish have one mouth and all squid have two eyes. Humans are perfect examples of unitary organisms. A life begins when a sperm fertilizes an egg to form a zygote. This implants in the wall of the uterus, and the complex processes of embryonic development commence. By 6 weeks the foetus has a recognizable nose, eyes, ears and limbs with digits, and accidents apart, it will remain in this form until it dies. The foetus continues to grow until birth, and then the infant grows until perhaps the 18th year of life; but the only changes in form (as opposed to size) are the relatively minor ones associated with sexual maturity. The reproductive phase lasts for perhaps 30 years in females and rather longer in males.

This is followed by, or may merge into, a phase of senescence. Death can intervene at any time, but for surviving individuals the succession of phases is, like form, entirely predictable.In modular organisms, or the other hand, neither timing nor form is predictable. The zygote develops into a unit of construction (a module, e.g. a leaf with its attendant length of stem), which then produces further, similar modules. Individuals are composed of a highly variable number of such modules, and their programme of development is strongly dependent on their interaction with their environment. The product is almost always branched, and except for a juvenile phase, effectively immobile. Most plants are modular and are certainly the most obvious group of modular organisms. There are, however, many important groups of modular animals and many modular protista and fungi.

In the growth of a higher plant, the fundamental module of construction above ground is the leaf with its axillary bud and the attendant internode of the stem. As the, bud develops and grows, it produces further leaves, each bearing buds in their axils. The plant grows by accumulating these modules. At some stage in the development, a new sort of module appears, associated with reproduction (e.g. the flowers in a higher plant), ultimately giving rise to new zygotes. Modules that are specialized for reproduction usually cease to give rise to new modules (although this is not true of all modular animals). The roots of a plant are also modular, although the modules are quite different. The programme of development in modular organisms is typically determined by the proportion of modules that are allocated to different roles (e.g. to reproduction or to continued growth).

Many ecological and evolutionary generalizations have been made in the past as if the unitary animal (such as a human or a mosquito) in some way typifies the living world. This is highly misleading. Modular organisms such as seaweeds, corals, forest trees and grasses dominate large parts of the terrestrial and aquatic environments.

Modular organisms may broadly be divided into those that concentrate on vertical growth, and those that spread their modules laterally, over or in a substrate. Many plants produce new root systems associated with a laterally extending stem: these are the rhizomatous and stoloniferous plants. The connections between the parts of such plants may die and rot away, so that the product of the original organism becomes represented by physiologically separated parts. The most extreme examples of plants 'falling to pieces' as they grow are the many species of floating aquatics like duckweeds (Lemna), and the water lettuce (Pistia). Whole ponds, lakes or rivers may be filled with the separate and independent parts produced by a single zygote.

Trees are the supreme example of plants whose growth is concentrated vertically. The peculiar feature distinguishing trees and shrubs from most herbs is the connecting system linking modules together and connecting them to the root system. This does not rot away, but thickens with wood, conferring perenniality. Most of the structure of such a woody tree is dead, with a thin layer of living material lying immediately below the bark. The living layer, however, continually regenerates new tissue, and adds further layers of dead material to the trunk of the tree. The majority of a tree is a ‘cemetery’ in which dead stem tissues of the past are interred, but the strength of the trunk solves the difficult morphological problem of obtaining water and nutrients below the ground, but also light perhaps 50 m away at the top of the canopy.

We can often recognize two or more levels of modular construction. The fundamental units of higher plants are assembled into clusters with a form that is itself continually repeated. The strawberry is a good example of this: leaves are repeatedly developed from a bud, but these leaves are arranged into rosettes. 'The strawberry plant grows: (i) by adding new leaves to a rosette, and (ii) by producing new rosettes on stolons grown from the axils of its rosette leaves. Trees also exhibit modularity at several levels: the leaf with its axillary bud, the whole shoot on which the leaves are arranged and the whole branch systems that repeat a characteristic pattern of shoots.

The growth of modular animals can be illustrated by a hydrozoan, like Obelia. Development begins when a short-lived, free-swimming planula larva attaches itself to a solid object. It gives rise to a horizontal root-like structure that bears a number of branched stalks. The basic Obelia modules, the polyps [            ] (which are both feeding and defensive structures), are borne on these stalks. The terminal polyp of each branch is temporarily the youngest, but is overgrown by the next one to develop, which arises as a bud at its base. The branched stalks remain as an interconnecting network between all the polyps in a colony. Reproduction in Obelia begins when tiny, free-swimming jellyfish are budded off from modified polyps called gonophores; these jellyfish then reproduce sexually to produce the dispersing planula larvae. Thus, these and similar animals, despite variations in their precise method of growth and reproduction, are as 'modular' as any plant. Moreover, in corals, for example, just like many plants, the individual may exist as a physiologically integrated whole, or may be split into a number of colonies—all part of one individual, but physiologically independent. The potentialities for individual difference are far greater in modular than in unitary organisms. For example, an individual of the annual plant Chenopodium album, may, if grown in poor or crowded conditions, flower and set seed when only 5O mm high. Yet, given more ideal conditions, it may reach 1 m in height, and produce 50 000 times as many seeds as its depauperate  [                     ] counterpart. It is modularity and the differing birth and death rates of plant parts, which give rise to this plasticity.

In fact, there is often no programmed senescence of the whole modular
organism—they appear to have perpetual somatic youth. Even in trees that
accumulate their dead stem tissues, or corals that accumulate old calcified
branches, death often results from becoming too big or succumbing to disease rather than from programmed senescence.                        

Ex.2. Comprehension check-up:


1. What are fundamental ecological facts of life?

2. What is the main aim of ecology?

3. In what case is it important for an ecologist to study the demographic processes (birth, death and migration) of individuals?

4. What is of great importance in unitary organisms?

5. Does the form of unitary organisms change greatly while they continue to grow? Is life form a predictable phase at each succession stage?

6. How do modular organisms differ from unitary ones?

7. What does their programme of development depend on?

8. In what way does a higher plant develop?

9. What is the programme of development in modular organisms typically determined by?

10. Which organisms: modular or unitary ones dominate environments?

11. How can modular organisms be divided?

12. What are rhizomatous plants?

13. Can you give examples of plants ‘falling to pieces” as they grow?

14. What peculiar feature distinguishes trees from most herbs?

15. How do trees develop?

16. What levels of modular construction is it possible to distinguish?

17. What is the typical process of growth for a modular animal?

18. Is individuality exhibited greater in modular organisms or in unitary ones?

19. Is it possible to say that a modular organism has a programmed senescence?



Ex.3. Translate the following words and word combinations into English:


Неоспоримый экологический факт, насекомое-вредитель, охраняемый участок, благоприятствовать рождаемости, приближать смерть, загрязненный ручей, различаемый нос, конечности с пальцами, не считая случайности, младенец, единственные изменения, фаза старения может наступить в любое время, смена жизненных фаз полностью предсказуема, с другой стороны, единица целого (модуль), наиболее показательная группа модулярных организмов, модули, которые отвечают за репродукцию, физиологически различные части, повторять характерную форму веточек, форма – определяющий фактор, прикрепиться к твердому объекту, стебли с ветвями, умереть от, отвердевшие ветки.


Reading B:   Life cycles and the quantification of death and birth



Ex.1. Read the following article and answer the question: Why do scientists need to construct life tables, survivorship curves and fecundity schedules?



Ex.2. Translate the first 2 paragraphs into Russian (in written form).



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