BIOLOGY OF SEAWEEDS OF ECONOMIC IMPORTANCE
191
in the following summer. Vegetative growth of the erect fronds has
been observed to continue into autumn. The papillate outgrowths
which bear the sexual organs commence development in the spring.
Regenerative growth has been observed from injured regions of the
thalli, and also from the damaged fruiting regions of the fronds.
Frequently this regenerative growth takes the form of a proliferation
of new frondage (similar to that described for FUCUS, p. 137).
Sexual organs only have been observed in the life-history of Gigartina, and the absence of tetrasporangia makes the sequence of events
in the life-history somewhat problematical. Both male organs and
carpogonia have been found in spring, summer and winter, but the
main period of reproduction appears to be in the summer months.
The cystocarps mature in July and August, and the main period of
spore discharge occurs between September and December. Subsequent
deterioration of the fronds follows the decay of the spent cystocarps.
The lack of tetrasporophytes suggests a reduction division occurring
in the carposporangium, but there is no convincing cytological evidence
for this. It appears that the spores give rise to young Gigartina plants
directly, with a disc-like primary stage in development (Marshall
et al., 1949; Powell, 1949). In experiments where spore settlements
were obtained on glass slides set out on the shore, the juvenile plants of
Gigartina were observed to grow under a film of diatoms and other
algae. In tests with a variety of substrata the young plants were
observed to favour quartzite grains. Fusion of neighbouring discs was
observed and the development of erect branches from around the edges
of the disc (see also sporeling development in Gracilaria, p. 197). The
first dichotomies by the apices of the young plants were observed 12
months after first settlement of the spores. Serious damage on the young
frondage was seen to result from the activities of browsing molluscs.
Experimental studies on the methods of harvesting and effects on
the standing crop have also been reported (Marshall et al., 1949). The
experiments were designed to study the effects of bad harvesting,
either by hand or by mechanical means, in which not only the frondage
but also the holdfast regions as well were removed. In most experiments it was found that hand-picking the fronds enabled an adequate
recovery of the crop, and after a sufficient time (12 months) the plants
on the " picked " areas were much the same in appearance as the
untouched controls, whereas experimental areas cut with shears failed
to show the same degree of recovery. Cropping by shears removes
most of the frondage, so that there is little protection for the young
developing branches.
Noticeable changes in the numbers and condition of the erect
191
in the following summer. Vegetative growth of the erect fronds has
been observed to continue into autumn. The papillate outgrowths
which bear the sexual organs commence development in the spring.
Regenerative growth has been observed from injured regions of the
thalli, and also from the damaged fruiting regions of the fronds.
Frequently this regenerative growth takes the form of a proliferation
of new frondage (similar to that described for FUCUS, p. 137).
Sexual organs only have been observed in the life-history of Gigartina, and the absence of tetrasporangia makes the sequence of events
in the life-history somewhat problematical. Both male organs and
carpogonia have been found in spring, summer and winter, but the
main period of reproduction appears to be in the summer months.
The cystocarps mature in July and August, and the main period of
spore discharge occurs between September and December. Subsequent
deterioration of the fronds follows the decay of the spent cystocarps.
The lack of tetrasporophytes suggests a reduction division occurring
in the carposporangium, but there is no convincing cytological evidence
for this. It appears that the spores give rise to young Gigartina plants
directly, with a disc-like primary stage in development (Marshall
et al., 1949; Powell, 1949). In experiments where spore settlements
were obtained on glass slides set out on the shore, the juvenile plants of
Gigartina were observed to grow under a film of diatoms and other
algae. In tests with a variety of substrata the young plants were
observed to favour quartzite grains. Fusion of neighbouring discs was
observed and the development of erect branches from around the edges
of the disc (see also sporeling development in Gracilaria, p. 197). The
first dichotomies by the apices of the young plants were observed 12
months after first settlement of the spores. Serious damage on the young
frondage was seen to result from the activities of browsing molluscs.
Experimental studies on the methods of harvesting and effects on
the standing crop have also been reported (Marshall et al., 1949). The
experiments were designed to study the effects of bad harvesting,
either by hand or by mechanical means, in which not only the frondage
but also the holdfast regions as well were removed. In most experiments it was found that hand-picking the fronds enabled an adequate
recovery of the crop, and after a sufficient time (12 months) the plants
on the " picked " areas were much the same in appearance as the
untouched controls, whereas experimental areas cut with shears failed
to show the same degree of recovery. Cropping by shears removes
most of the frondage, so that there is little protection for the young
developing branches.
Noticeable changes in the numbers and condition of the erect