190
A. D. BONEY
hyphae in the cortex and medulla of central regions of the frond gives
the midrib, a structure seen also in some species of the two related
genera, e.g. Gelidium pristoides, G. japonicum and Pterocladia lucidu
(Fan, 1961). Having similar modes of thallus construction to Gelidium
and Pterocladia, the same vigorous regenerative growth would appear to
be possible after harvesting.
10. Reproductive structures in Gelidium, Pterocladia and Xuhria
Detailed accounts of the reproductive structures of the three genera
have been given by Dixon (1968b) and Fan (1961). Tetrasporangia in
Gelidium and Pterocladia are formed from terminal cortical cells on the
lateral filaments. The mother cell enlarges and divides, forming four
spores arranged in a cruciate or tetrahedral pattern. The sporangia of
Suhria produce two binucleate spores per sporangium (bispores), and
similar structures are described for Gelidium pristoides.
Carpogonia are formed in either terminal or modified lateral
ramuli. The organs arise from lateral cells close to the main axis, and
the trichogynes project beyond the surface of the thallus. The carpogonium enlarges after fertilization and by subsequent amalgamation
growth gives rise to a large fusion cell from which the gonimoblast
filaments arise, and these in turn bear terminal carposporangia; the
developing carposporophyte is nourished by means of nutritive cells
formed after fertilization. A number of carpogonia in a fertile axis may
be fertilized at the same time, and the " cystocarp " which then develops
may be formed from a number of carpogonia.
Spermatangia arise in much the same way in all genera, but the
male plants are identical with the female plants and those bearing
tetrasporangia. The spermatangia; are formed from modified elongated
cortical cells, and the spermatia are abstricted from the apices.
11. Gigartina
Autecological studies on Gigartina stellata in the British Isles were
carried out by Marshall et al. (1949) in a detailed study of the seasonal
life of the plants. New frondage is formed early in spring, and becomes
prominent in May because of its different colour. As growth continues
the plants darken in colour, and the " cell rows '' which constitute the
cortex of the frond increase in number from the four to five cells in
spring to ten in early autumn. Cell wall thickness is also greater during
the autumn months. The erect fronds develop from a disc-like holdfast,
and are perennial structures once formed. The layering of the tissues
in the holdfast can be correlated with the annual growth of the plants,
since the early spring growth is more pronounced than that observed
A. D. BONEY
hyphae in the cortex and medulla of central regions of the frond gives
the midrib, a structure seen also in some species of the two related
genera, e.g. Gelidium pristoides, G. japonicum and Pterocladia lucidu
(Fan, 1961). Having similar modes of thallus construction to Gelidium
and Pterocladia, the same vigorous regenerative growth would appear to
be possible after harvesting.
10. Reproductive structures in Gelidium, Pterocladia and Xuhria
Detailed accounts of the reproductive structures of the three genera
have been given by Dixon (1968b) and Fan (1961). Tetrasporangia in
Gelidium and Pterocladia are formed from terminal cortical cells on the
lateral filaments. The mother cell enlarges and divides, forming four
spores arranged in a cruciate or tetrahedral pattern. The sporangia of
Suhria produce two binucleate spores per sporangium (bispores), and
similar structures are described for Gelidium pristoides.
Carpogonia are formed in either terminal or modified lateral
ramuli. The organs arise from lateral cells close to the main axis, and
the trichogynes project beyond the surface of the thallus. The carpogonium enlarges after fertilization and by subsequent amalgamation
growth gives rise to a large fusion cell from which the gonimoblast
filaments arise, and these in turn bear terminal carposporangia; the
developing carposporophyte is nourished by means of nutritive cells
formed after fertilization. A number of carpogonia in a fertile axis may
be fertilized at the same time, and the " cystocarp " which then develops
may be formed from a number of carpogonia.
Spermatangia arise in much the same way in all genera, but the
male plants are identical with the female plants and those bearing
tetrasporangia. The spermatangia; are formed from modified elongated
cortical cells, and the spermatia are abstricted from the apices.
11. Gigartina
Autecological studies on Gigartina stellata in the British Isles were
carried out by Marshall et al. (1949) in a detailed study of the seasonal
life of the plants. New frondage is formed early in spring, and becomes
prominent in May because of its different colour. As growth continues
the plants darken in colour, and the " cell rows '' which constitute the
cortex of the frond increase in number from the four to five cells in
spring to ten in early autumn. Cell wall thickness is also greater during
the autumn months. The erect fronds develop from a disc-like holdfast,
and are perennial structures once formed. The layering of the tissues
in the holdfast can be correlated with the annual growth of the plants,
since the early spring growth is more pronounced than that observed