480
N. BALAKRISHNAN NAIR AND M. SARASWATHY
in the cold waters of Holland and at Nieuport, Belgium (Anon, 1937 ;
Loppens, 1905) this timber has been attacked by Teredo when the
alkaloid was leached out, the outer alkaloid-free layer alone becoming
vulnerable to attack. The presence of alkaloids andother as yet undetermined poisonous inclusions in the tissues is believed to be responsible
for the unusual durability and natural resistance of many other timbers
such as Iron wood, Eusideroxylon xwageri and Bitter Angelims, Andira
vermifuga. Syncarpia laurifolia grown in Australia and in Hawaii lacks
resistant properties that give durability in sea water, borers attacking
both sapwood and heartwood. Edmondson (1947) regards the presence
of gum deposits as a specific factor in the comparatively high resistance
of Australian and Hawaiian timbers to marine borer attack. Resistance
shown by even the same species of timber is relative depending on such
factors as lack of age, rapidity of growth, place of origin, etc.
The resistance shown by Huon Pine, Dacrydiurn franklinii (Fitzsimons and Brooks, 1920) and teak, Tectona grandis (Money, 1811) is
due to the presence of obnoxious oils.
The Naval Research Laboratory of the U.S. Navy has been examining in the past years, the natural resistance of tropical woods to marine
borer attack (see Trussel and Jones, 1970). 114 different woods were
tested for a 14 month exposure. It was found that no timber available
in abundance and of sufficiently large size was found to show resistance
to all marine borer species. Dalbergia retusa (cocobolo) was highly
resistant in all environments but its potential as a commercial timber
is limited. Cordia aliodora, Chrysophyllurn cainito and Bouteria campechiana also showed evidence of resistance to marine borers over the
period tested. A few hard heavy woods such as Tabebuia guayacan,
Swartxiawe panamensis and Manilkara dariensis are apparently
capable of resisting tropical fungi. (See Trussel and Jones loc. cit.).
The brackish water shipworm Teredo healdi was found to be more
destructive than typical marine forms capable of boring into and
destroying even well known borer resistant timbers such as ekki,
greenheart and acapu.*
A. The role of silica
The significance of the presence of silica in the tissues of timber by
imparting durability to woods in underwater constructions, etc. has
been repeatedly stressed (Iterson, 1934 ; Gonggrijp, 1924, 1924a, 1932 ;
Record and Mell, 1924 ; Bianchi, 1932, 1933, 1934 ; Anon, 1938 ; Prison,
1942 ; Spoon and Loosjes, 1946 ; Amos and Dadswell, 1948 ; Amos and
Tack, 1952; Horn, 1948). Nanji and Shaw (1925) have shown that
nearly 90% of the total silica in plants is present as free silicic acid,
*See note added in proof, p. 509.
N. BALAKRISHNAN NAIR AND M. SARASWATHY
in the cold waters of Holland and at Nieuport, Belgium (Anon, 1937 ;
Loppens, 1905) this timber has been attacked by Teredo when the
alkaloid was leached out, the outer alkaloid-free layer alone becoming
vulnerable to attack. The presence of alkaloids andother as yet undetermined poisonous inclusions in the tissues is believed to be responsible
for the unusual durability and natural resistance of many other timbers
such as Iron wood, Eusideroxylon xwageri and Bitter Angelims, Andira
vermifuga. Syncarpia laurifolia grown in Australia and in Hawaii lacks
resistant properties that give durability in sea water, borers attacking
both sapwood and heartwood. Edmondson (1947) regards the presence
of gum deposits as a specific factor in the comparatively high resistance
of Australian and Hawaiian timbers to marine borer attack. Resistance
shown by even the same species of timber is relative depending on such
factors as lack of age, rapidity of growth, place of origin, etc.
The resistance shown by Huon Pine, Dacrydiurn franklinii (Fitzsimons and Brooks, 1920) and teak, Tectona grandis (Money, 1811) is
due to the presence of obnoxious oils.
The Naval Research Laboratory of the U.S. Navy has been examining in the past years, the natural resistance of tropical woods to marine
borer attack (see Trussel and Jones, 1970). 114 different woods were
tested for a 14 month exposure. It was found that no timber available
in abundance and of sufficiently large size was found to show resistance
to all marine borer species. Dalbergia retusa (cocobolo) was highly
resistant in all environments but its potential as a commercial timber
is limited. Cordia aliodora, Chrysophyllurn cainito and Bouteria campechiana also showed evidence of resistance to marine borers over the
period tested. A few hard heavy woods such as Tabebuia guayacan,
Swartxiawe panamensis and Manilkara dariensis are apparently
capable of resisting tropical fungi. (See Trussel and Jones loc. cit.).
The brackish water shipworm Teredo healdi was found to be more
destructive than typical marine forms capable of boring into and
destroying even well known borer resistant timbers such as ekki,
greenheart and acapu.*
A. The role of silica
The significance of the presence of silica in the tissues of timber by
imparting durability to woods in underwater constructions, etc. has
been repeatedly stressed (Iterson, 1934 ; Gonggrijp, 1924, 1924a, 1932 ;
Record and Mell, 1924 ; Bianchi, 1932, 1933, 1934 ; Anon, 1938 ; Prison,
1942 ; Spoon and Loosjes, 1946 ; Amos and Dadswell, 1948 ; Amos and
Tack, 1952; Horn, 1948). Nanji and Shaw (1925) have shown that
nearly 90% of the total silica in plants is present as free silicic acid,
*See note added in proof, p. 509.