The seeds of orchids (like those of some other saprophytic or semi-parasitic plants) contain a small embryo of only about 0.1 mm diameter, without any associated endosperm storage tissue. Upon germination, the embryo enlarges to form a small, corm-like structure, called a protocorm, which possesses a quiescent shoot and root meristem at opposite poles. In nature, a protocorm becomes green and accumulates carbohydrate reserves through photosynthesis. Only when it has grown and has sufficient stored organic matter does it give rise to a shoot and a root. Normal seedling growth then continues utilising the stored protocorm food reserves.
Bodies which, in their structure and growth into plantlets, appear to be identical with seedling protocorms (except that on synthetic media they may not be green), are formed during in vitro culture of different types of orchid organs and tissues. These somatic protocorms can appear to be dissimilar to seedling protocorms, and many workers on orchid propagation, have used terms such as 'protocorm-like bodies' (PLBs) to describe them.
When a shoot tip of an orchid is transferred to culture on a suitable medium, it ceases to grow and to develop as a mature shoot apex; instead it behaves as though it were the apex of an embryo, i.e. it gives rise to a protocorm (Vol. 2). Protocorm-like bodies also arise directly on some other orchid explants and proliferate from other PLBs in a fashion which is exactly comparable to the direct formation of somatic embryos.
Champagnat and Morel (1972) and Norstog (1979) considered the appearance of protocorms to be a manifestation of embryogenesis because they represent a specialised stage in embryo development and are normally derived directly from zygotic embryos. We think that this is the correct interpretation: in a previous edition of this book, protocorms were described under 'storage organs'.
Other protocorm-like structures. In vitro culture of small immature proembryos from developing barley seeds (Norstog, 1961, 1965a, 1970) or from the fern Todea barbara (De Maggio and Wetmore, 1961) has been noted to result in the formation of protocorm-like tissue masses from which root and shoots are regenerated after a period of irregular growth. Mapes (1973) recorded the appearance of such protocorm-like structures on shoot tips of pineapple, and Abo El-Nil and Zettler (1976) describe their direct formation on shoot tip explants of the yam Colocasia esculenta, or indirectly in subsequent callus cultures.
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