The following types of plant propagation have been discovered:
1. Sexual propagation
a) allogamy or xenogamy, outbreeding
b) autogamy, inbreeding
2. Vegetative, asexual propagation (apomixis).
Each of these mechanisms has proven to be adaptive in matching situations.
Allogamy, also called cross-fertilisation , is the usual and best-known way of sexual propagation. It guarantees genetic variability, and thus new combinations of alleles within a species. Allogamy is not restricted to flowering plants alone. Among spore plants, a type of allogamy using male gametes instead of pollen is common.
During the evolution of plants, mechanisms supporting allogamy and excluding self-fertilisation completely or almost completely evolved. Dioecism is the most important of them: male and female (+ and -) gametes develop separately on different individuals. Many spore and flowering plants are monoecious. Male and female gametes develop either on different parts of the parental plant or at different times. Fern prothalliums and anemophytous trees are typical examples. The original angiosperms has without doubt hermaphroditic flowers with stamens and carpels. They were self-incompatible. This is still a common state. More progressive angiosperms are characterized by monoecism or dioecism. Allogamy was thus supported further or even forced. In some genera, like Bryonia, for example, both dioecious and monoecious species occur. The latter are able to become dioecious by mutating. This type of dioecism is regarded as a secondary, derived feature.
In many angiosperm families, flowers are structured so as to prevent an accidental transfer of the pollen onto the stigma. The flower structure is often so elaborate, that only the aid of pollinators (insects, birds, etc.) guarantees a transfer of the pollen to the stigma of another flower.
The androecium and gynoecium of the flowers of some species mature subsequently. This is called dichogamy. In homogamy, in contrast, androecium and gynoecium mature simultaneously. In protandrous flowers, the androecium matures first, while in protogynous flowers, the gynoecium is the first to ripen.
In some genera, like Tulipa or Hyacinthus, tetraploid species are mostly self-compatible, while diploid species are self-incompatible. In some annual species, like Clarkia purpurea, species that can be crossed among each other occur, one of which is allogamous and can be pollinated by bees, the other of which is self-pollinating (H. and M. LEWIS, 1955). Besides the mentioned trees, especially perennial herbs are allogamous.
Autogamy is very rarely – in angiosperms never - the only mechanism of propagation. All species characterized as obligatory autogamous have a small share of allogamy maintaining a restricted gene flow between the populations and thus guaranteeing the unity of the species. The percentage of allogamy in Thlaspi alpestre, for example, is 5%.
Species, where autogamy is common but allogamy is not rare either, are said to be optionally autogamous. Optional autogamy is especially common in polyploids and during the initial colonization of new biotopes. Especially in successful pioneer plants like Chenopodium albatum, Avena fatura, Stellaria media, Oxalis corniculata, Lactuca serriola, Hordeum murinum, Rumex crispus, Raphanus sativus, Plantago lanceolata, and Capsella bursa-pastoris, the intraspecific variability at a new habitat is very limited. It is, nevertheless, considerable when comparing plants of different new habitats. The plants can spread, because they are the first colonizers. In subsequent periods of vegetation, pioneers are often substituted by other, more competitive species, a process called succession. Autogamous species are often annual. They have usually small, inconspicuous flowers that are not attractive for pollinators. Autogamy is useful with small numbers of individuals per area, since the safeguarding of the success of propagation is more important than the production of new genotypes. At the same time, autogamy is especially advantageous in the case of polyploids, since their genetic (the segregation of single alleles) is far more complex than that of diploid plants. In a diploid organism, a diploid gene can exist in the states AA, Aa, and aa. The situation is more complex in tetraploids: AAAA, AAAa, Aaaa, Aaaa, and aaaa. In such a situation, sexual propagation (allogamy) helps rarely to obtain more favourable constellations than inbreeding or vegetative propagation does.
I will treat this topic rather briefly here. Cell division is the most primitive type of propagation of organisms. It is predominant in procaryotes, protists, and single-celled algae. Plants have a high capacity of regeneration that enables them to propagate vegetatively. Parts of the shoot are usually able to root anew and to establish on in a new habitat. Flowering plants may not flower under certain conditions, but they keep nevertheless for years the ability to propagate.
It is well-known that many species like strawberries and many species growing on dunes develop specialized runners that serve propagation. Other species form specific fruit bodies or brood buds. The potato tuber or the brood bud of Bryophyllum calycinum are an example.
Vivipary is a vegetative phenomenon occurring, for example, in several grasses of the genera Poa, Festuca, and Deschampsia.
Agamospermy and vegetative propagation are collectively also called apomixis. Agamospermy is asexual seed formation. Three different types exist.
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Diplospory, where the diploid embryo-sac mother cell develops directly into an embryo, a process also called parthenogenesis. |
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In apospory, the embryo has its origin in a somatic cell from the surrounding of the embryo sac mother cell. |
In both cases a gametophyte develops, but meiotic division does either not take place or has no consequences. It is therefore also spoken of gametophytic apomixis. The third type of agamospermy is
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adventitious embryony. No gametophyte develops. The embryo stems from cells of the diploid sporophyte like the integument, instead. |
Apomixis is very common in angiosperms and pteridophytes, but was so far not detected in gymnosperms. It is conspicuously widespread in some grasses (Poa), Rosacea (Rubus, Sorbus), Compositae (Achillea, Crepis, Hieracium, Taraxacum), and Rutaceae (Citrus).
Due to the occurrence of apomixis in Hierarcium, G. MENDEL did not succeed in applying the laws he had discovered with Pisum sativum here. Genera where apomixis occurs are regarded as taxonomically difficult, because their species cannot be clearly distinguished, since they do often themselves consist of single clones. The situation becomes even more complicated, because apomixis can be either obligatory, facultative or sporadic, so that the complexity of the patterns of variation increases even more.
It seems that apomixis is extensively obligatory in most composite flowers, while it is facultative in Rosaceae and grasses, i.e. in these two families apomictically and sexually produced seeds occur side by side. The flower structure of a plant gives no indication whether or not apomixis occurs.
Most apomictic plants are polyploid hybrids, even though polyploidy alone does not promote apomixis and hybridization per se does not necessitate it.
Hybrids do often have difficulties with meiosis (see also the chapter about chromosomal numbers) leading to heavy losses of fertility. The selective advantage apomixis has in such situations is clear. It secures the existence of the hybrids and does at the same time offer a way out of the sterility problem. Hybrids and apomixis are often found in disturbed habitats. In contrast to autogamy that propagates inbreeding and homozygosis, apomixis stabilizes the status quo. The genotype of an individual remains the same in its progeny. Especially successful pioneer plants use genetic systems that are a compromise between a high rate of recombination and the stabilization of adaptive types. Facultative apomixis and hybrid formation proved to be an optimal combination.
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