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3 Order, transformation and emergence

3 Order, transformation and emergence

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Figure 3.11. The integration of

the genotype and the phenotype is

analogous to the BIOS in

computers. The BIOS integrates a

computer’s hardware and its

software. The BIOS is part burned

or flashed into a ROM and

read-only part included on ROM

chips installed on adapter cards,

and part on additional drivers

loaded when the system boots up.

Variations in the biological BIOS

can result in similar phenotypes in

plants with different genotypes or

different phenotypes in plants with

similar genotypes.

During the development of plants the growth ratios of single

organs and the structures in relation to the whole body usually

remain constant for certain periods, but there can be changes in

developmental timing, such as at the onset of flowering. It was Goldschmidt who first developed a clear idea of accelerations and retardations of certain gene-controlled developmental processes that are

due to quantitative differences, and their mutual interaction during

development. Thus, gene-control is implicated in the establishment

of growth gradients that are correlated to the size of the whole plant

body. During certain periods, an organ or a structure can grow more

quickly than the body as a whole (positive allometry), more slowly

(negative allometry), or with the same speed (isometry). Similarly,

one can refer to growth of a certain part of an organ in relation to

the whole (Figure 3.12).

Figure 3.12. Allometric changes

in leaf shape in Phyteuma

(Campanulaceae) from the base of

the plant (far left) towards the


Allometric growth can thus occur simultaneously at different levels within an individual, but this phenomenon can also be seen




Figure 3.13. Heteroblastic

development during ontogeny may

occasionally suggest the

recapitulation of phylogeny. Here

in Chamecyparis the juvenile foliage

seems to exhibit a kind of adult

foliage found in earlier stages of

cypress evolution.

between unrelated species. Large and small specimens of the same

species, or closely related large and small species will hardly ever

show the same type of proportions. By studying the different stages

of allometric growth in comparative studies of different species, rules

can be established that govern the correlation of body size and organs

in the course of phylogenetic differentiation, and the relative influence of selection processes. Such gradients of differentiation in body

proportions were ably demonstrated by the Cartesian transformations

of D’Arcy Thompson.

The idea that stages in the ontogeny of an individual may somehow afford clues to phylogenetic relationships was not as clearly

expressed in botany as in zoology, where the so-called biogenetic law

‘ontogeny is the short and rapid recapitulation of phylogeny’ was

given much currency by Haeckel in the late nineteenth century. It

was applied to botanical problems by Takhtajan who considered that

it was a theory that ‘penetrates botany with difficulty’. Alterations

that may affect the evolution of plants may, theoretically, occur at

any stage in the ontogeny of individuals, and, according to Takhtajan, the nature and extent of these alterations may be conceptualised

as four modes of change, i.e. prolongations, abbreviations, deviations,

and neoteny.

3.3.1 The developmental sequence

The developmental process even at its simplest level is very complex

and involves the communication between different elements, multiple hypercycles, and feedback loops. Consequently, there are relatively

few ways in which novel forms arise in development without a complete loss of integration. The idea of a developmental sequence of a

number of stages A → B → C is too simplistic. It is more like a web of

relationships that together fix a developmental process in space, but

nevertheless, this sequence provides a framework for the discussion

of developmental changes.

Addition of a stage in a sequence of development, what Takhtajan

called prolongations, is extremely common in plants, for example

pollen grains, seed coats, pericarps, and various parts of flowers, especially all types of outgrowths, such as the development of wings on

seeds and fruits. An addition is much more likely at the end of an

existing sequence of development, a terminal prolongation, than at

its beginning, or in the intercalation of a new stage because such a

pathway is less disrupted.

Addition is more common than subtraction or abbreviation.

Indeed, the complexity of developmental relationships, the network

of hypercycles, feedback loops and multi-dimensional influences,

make subtraction of part of a developmental process particularly

problematic. Abbreviations or subtractions are regarded as the omission of certain stages of development, the opposite of prolongations.

Vestigial structures are regarded as cases of terminal abbreviation.

Reduction in floral and vegetative parts, are examples of terminal



Expansion (A →b →C) or reduction (A →b →C) of a stage in a

sequence of development is much more frequent than a complete

deviation since it is less likely to disrupt the network of developmental relationships. Expansion or reduction can occur in space, expanding or reducing the size of an organ or part of an organ relative to

others. In many cases, loss of a developmental stage (subtraction) can

be shown to be no more than the reduction of that stage. Extension

(A→BB →C) or shortening are related to expansion or reduction but

occur in time. Extension of a particular stage of development often

results in expansion, whereas shortening often results in reduction.

If there are finite resources or limited time, extension of an earlier

developmental stage might result in a neotonous organ, resembling

an earlier stage of development (A →BC).

Takhtajan considered neoteny to be of prime importance, and

referred to it as ‘Peter Pan’ evolution. The term is used in a phylogenetic sense when it is considered that the ontogeny of a plant is

truncated, leading to a premature completion of development of the

whole plant or part of it. It is often considered synonymous with

‘paedomorphosis’ or ‘juvenilisation’. Neotenic changes depend on a

simplification or despecialisation of the phenotype. Takhtajan was

apparently swayed by the arguments of Koltzoff who pointed out that

such simplification does not affect the genotype. Subsequent mutation could then lead to an evolutionary radiation of forms with ‘new’,

juvenilised phenotypes.

Expansion/reduction or extension/shortening of development in

part of an organ results in a change in shape or orientation, what

Takhatajan called deviations. The profundity of the deviation depends

upon its timing, earlier deviations more profoundly influence later


Multiplication or combination of developmental sequences is particularly common in plants because of their repetitive (iterative) construction. Multiplication increases the number of organs or parts

of organs, normally by the division of a meristem, whereas combination results from the fusion of two or more neighbouring

meristems. Stebbins called this intercalary concrescence. Multiplication and combination/fusion in some cases are the consequence

of expansion/reduction or extension/shortening of developmental

stages. Extension of a developmental stage prior to meristem division

may not prevent multiple organs developing eventually, but often

results in them being fused together, whereas reduction or shortening of a developmental stage may lead to the early multiplication of

an organ.

Multiplication and combination are the two most profound means

by which a lineage can escape the developmental boundaries of an

autopoietic system, creating new evolutionary potential. It is fascinating that this occurs at different levels in the hierarchy of organisation. For example, the evolution of genes has occurred by exon

duplication (multiplication) and shuffling (combination). We describe

below several examples of multiplication and/or combination in the


or ginal




















additon at the end


additon at the begin


e xpansio









e xtensio


















duplication and modifcation










Figure 3.14. Possible different

modes of developmental changes

in the evolution of plants.




Figure 3.15. Two highly

reduced forms in the Lemnaceae:

(a) Lemna; (b) Wolffia.

evolution of reproductive organs. Is it stretching the point too far to

see the same process in speciation by geographic isolation (multiplication) and by hybridisation (combination), the two most profound

influences on plant speciation?

It probably is stretching the point too far to see the same processes in the development of ecosystems but perhaps not. Imagine

the multiplication of an ecosystem by the colonisation of an island

and the combination of different elements by this process and look

at the consequences in the development of novel island ecosystems

with changed relationships.

Multiplication and combination are such profound triggers for

evolutionary transformation because they can free an organ to function in a different way or adopt a novel function. Novel adapted organs

do not arise out of the blue, as fully integrated functioning systems

but normally arise by a transfer or change of function of an existing

organ. Describing this in terms of autopoiesis, it is this change of

function that so disturbs the network of developmental hypercycles

that a bifurcation point arises, releasing the organ from its earlier

developmental constraints. In this new developmental landscape evolution can be particularly rapid.

A word of caution is needed here. The classification of ontogenetic development into several different modes of change may not

be the most useful way to proceed in developmental studies. There

is every gradation between each mode, and obvious overlap, so that

confusion may result. In addition, it cannot be assumed that certain stages ever existed. The logic behind such a scheme is basically

typological and is the traditional thinking behind conventional ideas

of homology. By dividing phenomena into defined units in space

and time, for example leaf, shoot and root, it is too easy to makes

hypothetical assumptions about presences and absences. Because the

ontogeny of some plants may be conceptualised as serial steps, it

should not necessarily be applicable to all, or ad hoc hypothesis used

to explain discrepancies. It may be more useful to think of plant

development and evolution in a dynamic way, for example the process evolution of Sattler in which plants are examined throughout

their entire life cycle and with reference to a wider spectrum of plant


3.4 Macromutation and evolutionary novelty

Changes in developmental timing are frequently considered an important macroevolutionary process. It increases new possibilities in evolutionary terms. As Huxley pointed out ‘It is this possibility of escaping

from the blind alleys of specialisation into a new period of plasticity

and adaptive radiation which makes the idea of paedomorphosis so

attractive in evolutionary theory’.

Stebbins’ discussion of the role of what he calls intercalary concrescence fits in here. Abbreviation of a developmental stage in the


apical meristem or a prolongation of a previous stage can result in

the growing together of meristem primordia to result in union or

fusion of parts. This has been of profound evolutionary significance

in the evolution of reproductive structures, from the fused sporangia

of some pteridophytes to the astonishing diversity of floral structures.

Fusion of primordia that give rise to the same organ, connation, frequently results in tubular structures. Fusion of primordia that give

rise to distinct organs, adnation, gives rise to novel compound organs.

Both connation and adnation release evolutionary potential.

However, the acquisition of evolutionary novelty usually requires

a shift in ontogenetic development and, since this represents a saltatory step, it poses serious problems for defenders of a gradualistic

theory of evolution. The solution to the problem was perhaps provided by Darwin himself who pointed out that a change in structure

must simultaneously involve a shift of function. Severtsov was one

of the first to point out that an ‘intensification’ of function is all

that is needed for the adoption of a new function. In the course

of evolutionary change a morphological structure may have certain

additional characteristics that are, initially, selectively neutral, but

become increasing co-opted to perform new functions. Mayr stated

somewhat ambiguously that, ‘in most cases, no major mutation is

necessary in order to initiate the acquisition of the new evolutionary

novelty; sometimes, however, a phenotypically drastic mutation seems

to be the first step’. Is evolutionary novelty or macroevolution the outcome of macromutations or not? As a preliminary to an answer, it

may be constructive to consider the studies of evolutionary novelty

by Jong and Burtt in Streptocarpus.

3.4.1 Growth forms in the Gesneriaceae

The Old-World genera of the Gesneriaceae (Cyrtandroideae) are well

known for the unequal growth of their cotyledons (anisocotyly),

while many have rather atypical growth patterns, such as continuous growth of one cotyledon, and epiphyllous inflorescences. Saintpaulia is well known for its ability to regenerate from single leaves.

Atypical growth is found in a range of genera but species of Streptocarpus subg. Streptocarpus are the best known, and are all characterised

by the continuous growth of one cotyledon. This enlarged cotyledon

functions as a foliar organ but in many respects, it differs from a

true leaf. The term ‘phyllomorph’ was coined by Jong and Burtt to

distinguish the peculiar leaf-like structures of Streptocarpus from true

leaves and cotyledons (Figure 3.15). Jong further differentiated the

phyllomorph structure into a foliose component called the lamina

and its rooting petiole-like stalk called the petiolode. The inflorescence of most species usually arises from the base of the midrib of the


There are three groups based largely on the number of phyllomorphs they possess, and are either unifoliate or rosulate to varying degrees. In the unifoliate species of Streptocarpus, there is only




Figure 3.16. A Streptocarpus

species with a phyllomorph. Jong

differentiated the phyllomorph into

a foliose component called the

lamina and its rooting petiole-like

stalk called the petiolode.



one phyllomorph (the cotyledonary phyllomorph), and the plants are

monocarpic, whereas the rosulate species, which are perennial, have

been termed ‘colonial unifoliates’, and comprise few to numerous

repeating phyllomorph units.

In a study of S. fanniniae, Jong and Burtt found that there is

unequal growth of the cotyledons in the seedling. The larger cotyledon continues to grow from a basal meristem, and eventually is raised

above the level of the smaller cotyledon by intercalated tissue called

the mesocotyl that eventually differentiates as the cotyledonary petiolode. In S. fanniniae there is no plumule and the meristematic tissue

is sunk in an adaxial groove in the cotyledonary petiolode. It is from

this groove meristem of the petiolode that new phyllomorphs and

the inflorescence subsequently develop, while roots grow from the

abaxial surface. This groove meristem is functionally the equivalent

of the conventional apical meristem. In the regions at the base of

each lamina lobe, and where the petiolode merges with the lamina

there remain intercalary meristematic tissues called the basal and

petiolode meristems, respectively. The basal meristem is responsible

for continued growth of the lamina, whereas the petiolode meristem

is responsible for growth of the midrib, and the elongation of the

petiolode. In some species, part of the lamina dies back during an

unfavourable dry season, and a zone of abcision is clearly recognisable. Growth of the lamina recommences in favourable conditions

with activity in the basal meristem.

The petiolode has, by its possession of gaps in the vascular cylinder, and roots, a shoot-like nature. In contrast, its dorsiventrality

and terminal lamina suggests a petiole. Although the phyllomorph

is interpreted as a basic unit of structure combining features of both

leaf and shoot, and applicable to the Gesneriaceae, similar ideas have

been expressed by Sattler, and Arber. Jong and Burtt state that the

coordinated activity ‘gives the phyllomorph the stamp of distinction,

a morphogenetic innovation that has provided new possibilities in


the evolution of form’. ‘Steptocarpus might, indeed, seem to exhibit

the acme of neoteny in flowering plants’.

In some species of Chirita and Streptocarpus subg. Streptocarpella

flowers are occasionally produced by the cotyledon, suggesting that

the evolution of the unifoliate condition evolved from ancestral plants

that could develop facultatively in this way. In Streptocarpus nobilis, unifoliate growth is also facultative. Hilliard and Burtt speculated on the

affect of genetic change on the development of the plumular bud in a

caulescent plant that already possessed an accrescent cotyledon, and

suggested that the unifoliate habit could develop in this way. Central

to this argument is the role played by the environment. The unifoliate habit is well suited to relatively unoccupied habitats such as steep

banks in forest or sheltered cliff faces, or on mossy tree trunks. If such

novelty can arise from facultative ability (i.e. the ability to be flexible

or plastic) then perhaps we should focus our attention of the genetics of phenotypic plasticity rather than macromutations. It should be

recalled that small genotypic changes might produce massive phenotypic effects. Perhaps, by making a contrast between micromutations

and macromutations we are indulging in semantics, with the result

that we ask the wrong questions.

3.5 Unity and diversity; constraint and relaxation

One of the most compelling characteristics of organisms, and one that

we often take for granted, is that no two individuals are alike. Each

one is uniquely different yet, at the same time recognisable as belonging to a species. Individuality is the hallmark of all species though

the distinction between individuals is obscured in clonal organisms

such as plants. With their remarkable ability to reproduce asexually,

plants can produce separate individuals that are genetically identical.

Organisms are biologically constrained to conform to relatively

standard physical form and behaviour, and yet have an almost limitless diversity of individual appearances and behavioural preferences

(novelty). These two processes are the stuff of evolution. One provides

continuity over time, while the other provides the basis for evolutionary change. This continuity, this faithfulness to already-existing form

is often considered deterministic, i.e. the organism develops according to a ‘blueprint’ encoded in the genes. Individual variation is then

regarded as the outcome mainly of minor genetic variation passed

on in the genes from parent to offspring and chance (stochastic) processes operating within the growing body and in the environment.

Minute random variations in the internal or external environment

may have a profound effect on the growing individual. However, novelty may not necessarily be the outcome of stochastic processes alone.

It may be just one possible result of a dynamic spectrum of possibilities involving deterministic processes both at the individual and

organismic levels of evolution.






Figure 3.17. Predictable

architectural forms in

non-flowering plants that follow a

determinate pattern of growth: (a)

lycopod; (b) Araucaria heterophylla.

The expression of a particular genotype in a particular environment is called the phenotype. Deterministic factors ultimately have a

genetic basis, and are responsible for the conformation to the physiological and morphological parameters of the species. Nevertheless,

environmental or opportunistic factors impinge on plant form and

the ecological requirements of a species. Both factors combine to

give a historical trajectory to a species (heredity). It is from the relative contributions of these contrasted processes (genotype and phenotype), and their interaction with the environment that plant form

emerges. The way plants develop varies widely and, in the majority

of species, there is a norm of reaction, i.e. there is a spectrum of possible outcomes. Ability to respond to chance environmental variables

is ultimately determined by genotype, and differs widely in different

groups of plants, in closely related species, and even between individuals of the same species. Such responses also vary at different times

in the life cycle. In some species with more precise growth, development appears to be more deterministic; for example certain palms

and arborescent monocots usually cannot respond to crown damage

because they lack lateral meristems. Predictability of vegetative form

can be shown to be widespread in non-flowering plants from lycopods

to conifers. In other groups of plants, predictability of final form is

more elusive. This stochastic response is usually expressed by reiterative vegetative growth and is one of its most important adaptive

features of plants, but it can only be assessed after the initial deterministic component of growth has been recognised. Thus, it may be

impossible to predict how any individual plant will develop in a given


In marked contrast to vegetative parts, floral and fruit morphology

are highly deterministic, with little scope for opportunistic development, which is one reason why these structures have lent themselves

to a typological classification, and are so important for plant systematics. Naturally, there are differing views as to the extent to which

plant form is determined by their genes or by chance events produced

by the interaction of the genotype with the environment.

3.6 The phenotype

3.6.1 Developmental reaction norms

The phenotype of a plant is the sum total of its observable characteristics, and is the outcome of a complex relationship between the

genetic coding of the individual (the genotype) and the environment.

The relationship between phenotype and environment is referred to

as the norm of reaction. How this harmonisation process works on

the ‘laws of growth’ is still largely obscure, but certainly it is manifest through the normal developmental processes of the plant. Therefore, it is more appropriate to talk about a ‘developmental reaction

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