Protocorm formation in orchids

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...