The Formation and Ultrastructure of Rhizoids on Protonemata of Dryopteris borreri Newm

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Abstract
The fern gametophyte rhizoid appears as a highly differentiated cell, probably concerned with anchorage and absorption, with many of the features of other elongate plant cells such as root hairs and pollen tubes. This paper describes the formation and ultrastructure of the rhizoids produced early in the orderly sequence of cell division and differentiation in young protonemata grown on liquid culture medium. The elongating first rhizoid contains an apical cytoplasmic mass, the anterior part of which is devoid of plastids but contains many small vesicles and Golgi bodies. Three classes of vesicle are present, two of which appear to be derived directly from the Golgi apparatus. Vesicles of the third type resemble ‘coated vesicles’ and may be derived from one of the other two vesicle types. The remainder of the cytoplasm, excluding a basal region close to the prothallial cells, appears to be degenerate. This suggests that growth takes place at the apex as in rhizoids of other ferns. The structure of the wall of a rhizoid cell differs from that of a prothallial cell wall. The latter has an orientated fibrillar structure whereas the former is composed of a random network of fibrils within which electron dense deposits occur. This difference in structure is discussed in relation to the different but complementary functions of the colourless non-dividing rhizoids and the photosynthetic and meristematic protonemal cells.