Control Systems in the Living World - Building the Most Complex Structure on Earth - page 42

The Plant Bauplan and Control System
The modern concept of the modular organization of plants by repetition of the same
elementary units owes its origin to the observation of the founder of the discipline
of morphology, Johann Wolfgang von Goethe (1749-1832), that flowers and fruits
are repetitions of the foliage that “differ only in degree and not in kind.” Goethe
best expressed this observation in his famous dictum: “All shapes are similar, yet all
unlike” ( Alle Gestalten sind ähnlich, und keine gleichet der andern ), which appears
at the beginning of his poem on the Metamorphose of Plants ( Metamorphose der
Pflanzen ) (1798) ( Nickelsen, 2002 ).
We do not know whether, or how much, knowledge of the control system in ani-
mals can help reveal the plant's integrated control system. And this is for a good
reason: essential differences exist in the Bauplans of both groups, and the degree of
complexity and diversity of the animal structure is obviously greater. There are only
three organ systems (root, shoot, and reproductive systems) in plants, whereas 11
organ systems (nervous, digestive, cardiovascular, urinary/excretory, muscular, skel-
etal, respiratory, reproductive, endocrine, immunitary, and integumentary systems)
have evolved in higher animals.
At one time, plants were thought to consist of five morphological units (roots,
stem, leaf, floral organs, and ovules), but the prevailing view now is that plants con-
sist of only three units (roots, stem, and leaf) and according to the “heterodox” con-
cept, the shoot and leaves are not distinct units; the leaf is seen as a partial shoot
in the sense that “Each part is in essence an image of the whole” ( Lacroix et al.,
2005 ) and the plant itself is “a repetitively branched system alternatively composed
of leaves and shoots, each being a shoot in different degrees of wholeness” ( Classen-
Bockhoff, 2001 ).
The number of architectural models in living plants is very small (23), and far less
than would theoretically be predicted by the possible combinations of the elemen-
tary units ( Barthélémy and Caraglio, 2007 ). The strict modularity of the plant archi-
tecture may be responsible for the majority of the relatively limited morphological
inventiveness in the kingdom Planta, compared to animals ( Figure 1.25 ).
Unlike animals, where adulthood marks the end of the development of tissues and
organs, plants continue to produce new organs throughout their life. So while the
purpose of the control system in animals is to maintain the structure and function
of the organism upon reaching adulthood, in plants, it is also to produce new organs
during the entire life.
Regardless of surprises that future research may bring, the modular organiza-
tion and relative simplicity of plant structure, function, and behavior compared
to animals, would tempt a biologist to predict that plant evolution may not have
required a control system of the complexity and sophistication of that found in
animals.
In the present fragmentary picture of plant control, hormones represent the prin-
cipal extracellular signals and the higher step in the known hierarchy of control in
plants. This is substantially different from what we observe in animals, from lower