Transitional Forms from Embryo to Bud

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The analysis of literature and the original data on the development of sexual and somatic embryos in natural conditions and in vitro has led us to conclude, that there are structures which differ from typical sexual and somatic embryos and from bud by their morphology. In connection with this we introduce now a term "transitional form" (in the terms of evolution) which means a structure exhibiting traits of an embryo (for example, globular, heart- and torpedo-shaped stages of development) and of a bud (formation of adventitious roots during regeneration) [5, 12, 37]. The embryos of Nelumbonaceae (Nelumbo nucifera), Ceratophyllaceae (Ceratophyllum demersum), Poaceae, Orchidaceae and Orobanchaceae illustrate the possible ways of such transition. A similar phenomenon can be observed among somatic embryos.

The embryos of Nelumbo (hydrophyte) and Ceratophyllum (hydatophyte) lack epiphysis and hypophysis. In the embryo of Nelumbo the main root is substituted by the adventive roots during germination. The latter originate at the base of plumule leaves at the later stages of embryogenesis. However, the embryo of Nelumbo is bipolar from the very beginning of its development [38, 39].

The embryo of Ceratophyllum seems to be bipolar at the first stages of its genesis, as it exhibits a group of cells which can be taken for initials of a main root. However, in the mature embryo only a well developed plumule can be found, and no main root. As for the adventive roots, they do not arise neither in the seed, nor in the seedling. The seedling of Ceratophyllum, thus, lacks typical bipolarity [40, 41].

In the majority of Poaceae (xerophytes) the mature embryo exhibits well formed plumule and developed adventive roots (their number depends on species, where and when the parental plant grows). The main root had been transformed into coleorhiza in the course of evolution [42-45]. However, a number of investigators are of the opinion that the main root exists in the grasses embryo. The embryo is bipolar and dorsoventral from the first stages of development, although the question remains, whether the grasses embryo preserve the primary polarity or it is replaced by a secondary polarity in the course of embryogenesis.

In Orchidaceae the shoot and root apices are not exhibited morphologically in the mature embryo. Later during protocorm formation the bud and the adventive root arise and the secondary polarity establishes [9, 46, 47].

The embryos of parasitic plants can serve as best model for investigation on reduction of typical initials of epiphysis and hypophysis of shoot and root apices (Fig. 10). A considerable peculiarity of its genesis is great variability of the first developmental stages. It is displayed in the diverse contribution of ca and cb derivatives into the formation of embryo body. For example, in Aeginetia indica even the first few divisions are irregular, so that the type of embryogenesis can not be elucidated.

1. Capsella bursa-pastoris (Onagrad-type), mesophyte (after Maheshwari, 1950)

2. Nelumbo nucifera (Asterad-type), hydrophyte (after Titova, Batygina, 1996)

3. Ceratophyllum demersum (Asterad-type), hydatophyte (after Shamrov, Batygina, 1984, 1988)

4. Triticum aestivum (Graminad-type), xerophyte (after Batygina, 1987)

2. Nelumbo nucifera (Asterad-type), hydrophyte (after Titova, Batygina, 1996)

3. Ceratophyllum demersum (Asterad-type), hydatophyte (after Shamrov, Batygina, 1984, 1988)

Asterad Type

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