84 
identifying the included insects in this pale amber, 
came to the conclusion “ that this light amber (or 
copal) is of very recent origin, not earlier than 
Pleistocene, and contains a fauna which doubtless 
consists mainly (at least) of species still living.’”’ The 
point of great interest to me is that among these 
representatives of still living species he found “‘a 
small bee which seems not to differ at all from the 
common living Trigona leviceps Smith.” 
Now Trigona leviceps, or, as it is sometimes called, 
Melipona leviceps, is the actual, or at least chief, 
source of the common resinous substance known 
as Dammar (Burmese—Pweée-nyet). These bees build 
in hollows within trees, crevices among rocks, etc., 
and line the interior surfaces of their nests with 
a massive resinous substance. This resinous sub- 
stance is the common Dammar of the Burmese 
bazaars, and is used largely by the Burmese for the 
caulking of boats. Hooper (Rep. Labor. Ind. Mus., 
1904-5, 23-4) reports on two samples examined by 
him. It seems to be the general opinion that it is 
largely constructed by the bees from the oil and 
resin of Dipterocarpus. 
It seems to me, therefore, that the light amber 
beads examined by Prof. Cockerell may quite likely 
be fossil Dammar, or in other words, Dammar de- 
posited in crevices and holes in the earth or rocks 
by Melipona, which has afterwards been buried up and 
entombed and fossilised. The inclusion of a specimen 
of the actual bee in this fossil Dammar would be not 
only possible but highly probable if such is the case. 
Although I do not know the place from which the 
pale Chinese amber comes, I offer the above suggestion 
as to its origin; and from what I saw of the actual 
occurrence of the Burmese amber in the Hukong 
Valley amber mines it seems to me not unlikely 
that some such method may be the explanation of 
its origin also, though in this case one would not 
expect the depositing insect to be the same species 
as that depositing Dammar at the present day. 
MuRRAY STUART. 
Indo-Burma Oilfields, Ltd., Thayetmyo, 
Burma, December 8. 

Modern Psilotacez and Archaic Terrestrial Plants. 
WitH the establishment of an early Devonian 
group of vascular cryptogams showing fundamental 
resemblances with the modern Psilotacee, the con- 
troversy over the essentially primitive or reduced 
nature of the latter family may be said to be closed. 
In spite of important points of difference, the 
resemblances appear to suffice to link the Psilotacee 
with the most archaic types of terrestrial plants of 
which the structure is known at all adequately. 
The object of the present note is to record another 
piece of evidence pointing in the same direction. 
As Kidston and Lang say, “In its anatomy 
Asteroxylon is most closely comparable with the 
Psilotacee and with Lycopodium ”’ (‘‘Old Red Sand- 
stone Plants,’ etc., Part III., 1920, p. 667). So 
far as the leafy shoot is concerned, however, the 
hollow stele of the Psilotaceze, as generally described, 
does not readily fall into line with that of Asteroxylon 
or Lycopodium, in which the centre is occupied by 
a more or less compact mass of cauline xylem. It 
is therefore of some interest to note that one or 
more cauline strands devoid of protoxylem are 
normally present in the pith of Tmestpleris Vieillardi 
Dang., an erect terrestrial form said to be endemic 
in New Caledonia. The medullary strands generally 
arise from the ring of peripheral strands in the 
transitional region between the rhizome and aerial 
shoot, and when traced distally as a rule end blindly 
NO. 2777; VOL. Tit] 
NATURE 
[ JANUARY 20, 1923 
in the pith, although they sometimes merge into the 
peripheral strands. They show a good deal of 
variation in the degree of their development, generally 
not extending very far up into the leafy shoot, but 
their presence is a normal feature of the anatomy. 
In this respect, therefore, Tm. Vieillavdi facilitates 
the comparison of the Psilotacezee with the Devonian 
genus Asteroxylon, and also serves to strengthen 
their lycopod affinity, already established on other 
grounds. In a paper read before the Cambridge 
Philosophical Society (see NATURE, June 13, 1918, 
vol. 101, p. 299) I directed attention to this and 
other features, in view of which I regarded Tm. 
Vieillardi as the most primitive member of the 
Psilotacee ; but the discovery of Asteroxylon adds 
point to the conclusion there arrived at. It is 
natural to regard the poorly developed and variable 
medullary xylem of Tm. Vieillardi as a stage in the 
disintegration of a once continuous and solid cylinder 
of cauline xylem extending throughout the length 
of the axis; the hollow steles of Tm. tannensis and 
of Psilotum would ‘thus form the next stage in the 
reduction. This reduction within the group, how- 
ever, need not affect the essentially primitive nature 
of the Psilotacez as a whole. 
I must add that medullary xylem had previously 
been recorded in Tmesipteris on two occasions: (i.) 
by C. E. Bertrand, 1885, ‘‘ Recherches sur les 
Tmésiptéridées,”’ p. 248, Fig. 215 (A); and (ii.) by 
P. A. Dangeard, Le Botaniste, 1890-91, p. 17, Pl. 
XI. Fig. 1. But the nature of the material at 
their disposal (herbarium specimens) appears to have 
precluded a detailed investigation by the French 
authors ; they make only a passing reference to the 
feature in question, apparently considered by them 
to be only a rare occurrence. B. SAHNI. 
Botany Department, 
University of Lucknow, India, 
December 7. 

Action of Cutting Tools. 
Ir is true, as Prof. Andrade points out in NATURE 
of December 30 (vol. 110, p. 876), that I am “ not 
altogether familiar with the work that has already been 
done on the subject’’: indeed itis obvious. And after 
glancing through the 82 pages of bibliography at the 
end of Prof. E. C. Bingham’s ‘‘ Fluidity and Plasticity,” 
I feel certain that I shall remain in this state. Tresca’s 
Memoirs, however, are very well known among 
engineers, and they have been quoted and digested 
by several writers of engineering text- books and 
papers, but it is doubtful if the practical use of cutting 
tools has been much influenced by Tresca’s work, — 
beautiful and interesting though it is. 
The important problem which faces the user of 
cutting tools is the preservation of the cutting edge 
under heavy loads, and while Taylor’s work is the 
outstanding contribution on the matter, Mr. Mallock’s 
explanation of the influence of friction on the upper 
surface of the tool is invaluable to the machinist. 
The study of the shaving, while perhaps uninteresting 
to the physicist, is vitally important to the engineer, 
for, on one hand, the machinist watches the be- 
haviour of the tool very much in the shaving, and, 
on the other, the plastic flow on the back of the 
shaving, which produces flat-backed shavings from 
round-nosed tools, is probably a principal cause of 
the undesirable heating of the tool, and an important 
factor in the frictional] phenomena involved. 
H. S. Rowe tt, 
Director of Research. 
Research Association of British Motor and 
Allied Manufacturers, 
15 Bolton Road, W.4, January 4. 
