THE BIOLOGY OB WOOD-BORING TEREDINID MOLLUSCS
481
probably in a colloidal state whereas the other 10% is probably in
combination with a polysaccharide constituent of the plant in a form
only t o be extracted after treatment with acid. According to Gonggrijp (1932) the capacity of woods to repel marine borers seems to be
correlated with the high silica content together with the compactness
of the tissue. Timbers possessing exceptional powers of resistance to
marine boring organisms have been reported from the Pacific Islands,
Indonesia, Australia, Caribbean and South America. A majority of the
700 species examined by Prison (1 942) from Belgian Congo contained
more than 0.50% of silica which is considered as the minimum amount
effective against boring animals. Reports by Amos and Dadswell (1948)
and the Institute of Paper Chemistry (1951) give an idea of the amount
of silica present in timbers. The available data strongly suggest that
silica, if present in sufficient quantity, may be an important factor in
protection against shipworms.
B. Bark of trees
That the intact bark on piling is an effective temporary chemical
barrier against shipworm attack has long been known (Jarvis, 1855;
Jarvis, 1948; Mobius, 1875; Buchanan, 1877). This is owing to the
presence of appreciable amounts of silica, alkaloids, tannins or other
substances which offer considerable resistance t o shipworms. The
underlying wood, especially the deep inner layers, is devoid of effective
concentrations of these defensive chemical components and is, therefore, highly susceptible t o attack. Thus the turpentine timber used in
Australian waters (Johnson, 1932; Johnson et al., 1936; Rust and
Ferguson, 1935; Jarvis, 1948), the American hemlock (Bate and Westwood, 1863-68), Beech trunk in the Bay of Aabenraa, Denmark (Mobius,
1875), Prickly tea tree in N.S. Wales (Roughley, 1925), Oak piles at
Portland, Maine (Jaeger, 1936), Swamp Mahogany in Australia (Boas,
1947) and Sen timber in Yokohama, Japan (Mawatari and Kobayashi,
1952) are all known to offer greater resistance to the ravages of shipworms with the bark intact than without it. Many authors have noted
that shipworms will not penetrate wood through the barrier of the bark
(Holbrook, 1902; Cameron, 1909; Hamer, 1920; Foxworthy, 1921 ;
Ilsley, 1921; M’Gonigle, 1925; Neily et al., 1927; Kramp, 1937; Edmondson, 1955 ; Nair, 1956a). The principle behind using green piles is
to retain the bark without damage and allow it t o adhere to the wood
as firmly as pospible during the piling operations (Walsh, 1920), since
the value of the bark depends not only on its character and composition but also upon its close adherence t o the sapwood. Experts
have, therefore, recommended all possible ways and means for retaining
481
probably in a colloidal state whereas the other 10% is probably in
combination with a polysaccharide constituent of the plant in a form
only t o be extracted after treatment with acid. According to Gonggrijp (1932) the capacity of woods to repel marine borers seems to be
correlated with the high silica content together with the compactness
of the tissue. Timbers possessing exceptional powers of resistance to
marine boring organisms have been reported from the Pacific Islands,
Indonesia, Australia, Caribbean and South America. A majority of the
700 species examined by Prison (1 942) from Belgian Congo contained
more than 0.50% of silica which is considered as the minimum amount
effective against boring animals. Reports by Amos and Dadswell (1948)
and the Institute of Paper Chemistry (1951) give an idea of the amount
of silica present in timbers. The available data strongly suggest that
silica, if present in sufficient quantity, may be an important factor in
protection against shipworms.
B. Bark of trees
That the intact bark on piling is an effective temporary chemical
barrier against shipworm attack has long been known (Jarvis, 1855;
Jarvis, 1948; Mobius, 1875; Buchanan, 1877). This is owing to the
presence of appreciable amounts of silica, alkaloids, tannins or other
substances which offer considerable resistance t o shipworms. The
underlying wood, especially the deep inner layers, is devoid of effective
concentrations of these defensive chemical components and is, therefore, highly susceptible t o attack. Thus the turpentine timber used in
Australian waters (Johnson, 1932; Johnson et al., 1936; Rust and
Ferguson, 1935; Jarvis, 1948), the American hemlock (Bate and Westwood, 1863-68), Beech trunk in the Bay of Aabenraa, Denmark (Mobius,
1875), Prickly tea tree in N.S. Wales (Roughley, 1925), Oak piles at
Portland, Maine (Jaeger, 1936), Swamp Mahogany in Australia (Boas,
1947) and Sen timber in Yokohama, Japan (Mawatari and Kobayashi,
1952) are all known to offer greater resistance to the ravages of shipworms with the bark intact than without it. Many authors have noted
that shipworms will not penetrate wood through the barrier of the bark
(Holbrook, 1902; Cameron, 1909; Hamer, 1920; Foxworthy, 1921 ;
Ilsley, 1921; M’Gonigle, 1925; Neily et al., 1927; Kramp, 1937; Edmondson, 1955 ; Nair, 1956a). The principle behind using green piles is
to retain the bark without damage and allow it t o adhere to the wood
as firmly as pospible during the piling operations (Walsh, 1920), since
the value of the bark depends not only on its character and composition but also upon its close adherence t o the sapwood. Experts
have, therefore, recommended all possible ways and means for retaining