JUNE 20, 1912] 
NATURE 
399 
, 
ELEREBRS TO THES EDITOR. 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice is 
taken of anonymous communications.] 
The Effect of Grass on Plants. 
In a review of the thirteenth report of the Woburn 
Fruit Farm which appeared in Nature a short time 
ago, special reference was made to some of our ex- 
periments which seemed to prove conclusively that 
the injury done to trees by grass growing above their 
roots must be due to something excreted from, or 
resulting from the growth of, the grass, and not to 
its abstracting anything from the soil, or interfering 
mechanically or physically with the tree roots. In 
these experiments the trees were grown in plots of 
soil or sand, on which rested pans of soil or sand 
with grass growing in them. The pans had per- 
forations in the bottoms covered by fine wire gauze. 
The trees used for comparison had, of course, similar 
pans placed above them, but without grass growing 
in them. The deleterious effect of the grass in these 
circumstances was nearly as great as when it was 
growing in the medium in actual contact with “he 
roots. 
These experiments are now being repeated on plants 
other than fruit trees, namely, tobacco, tomatoes, and 
barley; the plants are in every case growing in soil, 
but the pans contain soil in some cases, and sand 
in others. Where they contain soil the effect of the 
grass growing in them has been most marked, 
especially on the tobacco, where the plants are not 
one-quarter the size of those without grass; where 
the pans contain sand the effect has been much less, 
being noticeable chiefly by the paleness of the plant 
leaves, rather than by the stunting of growth. This 
indicates that the toxic effect varies considerably with 
the nature of the medium in which the grass is grow- 
ing, and harmonises with previous observations that 
the effect of grass on trees varies considerably with 
the nature of the soil. With barley no certain effect 
of grass has yet been noticed, and it is quite possible 
that grass may not be deleterious to plants of the 
same order as itself. 
It was observed that in all cases the plants with 
grass above them appeared just at first to do rather 
better than the others. This is consistent with other 
observations on this subject, and also with the recog- 
nised stimulating effect of toxins in minimal doses. 
These experiments have not been completed, but the 
publication of a note on them may give others the 
opportunity of repeating them during the present | 
season. SPENCER PICKERING. 
The Local Races of Burchell’s Zebra. 
In Nature for June 6 (p. 364) there is a summary 
of a paper on zebras by Major Stevenson Hamilton, 
which was read before the Zoological Society on 
May 21. The author pointed out that it was possible 
to shoot in one herd in the Transvaal specimens 
exhibiting features claimed to be distinctive of such 
races as E. burchelli wahlbergi, E. b. transvaalensis, 
and E. b. chapmanni. From this circumstance Major 
Stevenson Hamilton concluded that the subspecies or | 
local races in question had been based upon inadequate 
museum material. 
Presumably the zebras observed, since they were 
shot in the Transvaal, belonged to the race named 
transvaalensis. It is not surprising therefore that 
they presented the characters of that form. Moreover, 
since the Transvaal lies between the areas of South 
Africa occupied respectively by E. b. wahlbergi and 
NO. 2225, VOL. 89| 
=) 
E. b. chapmanni, the occurrence of zebras there show- 
ing features possessed by those two subspecies is 
precisely what one would expect. For the subspecific 
rank assigned to the two forms in question implies 
the known, or expected, existence of intermediate 
forms in an intermediate geographical area. 
Hence the value of Major Stevenson Hamilton’s 
contribution to the question at issue lies in the proof 
it supplies, not of the unsoundness, but of the sound- 
ness, of the conclusions reached by museum 
systematists, at all events so far as the races of 
zebras under discussion are concerned. 
Zoological Society, June 12. Re IeeBococs 
Boulder Clay in Essex. 
THE extensive deep sewerage works now being 
carried out under Mr. H. ‘looley for the Essex 
County Council at Harlow have disclosed facts of 
considerable interest to students of glacial geology. 
The main sewer from Potter Street cuts through the 
hill of Boulder Clay between that place and Harlow 
at depths ranging up to 32 ft. The excavations and 
tunnels are entirely in the Boulder Clay, which 
assumes here an extraordinary till-like character, 
more so than in any exposure which has come under 
my observation in southern England. It is a black 
(rather slimy) clay, such as may well have been 
derived from the pounding up of Kimmeridge Clay, 
or Oxford Clay (as the latter is worked at the exten- 
sive works of the London Brick Company at Fletton, 
near Peterborough). Through this numerous chalk 
fragments are dispersed, and in the lower portions 
boulders (rounded, subangular, angular, and often 
beautifully striated) are met with in great quantity. 
Among the erratics rocks have been recognised from 
the Carboniferous Limestone (abundant), the Roth- 
liegendes, the Magnesian Limestone series, the 
Bunter (pebbles), the Lias, the Great Oolite, the 
Oxford Clay (by fossils), the Kimmeridge Clay (by 
fossils), the Chalk and the Eocene (sarsens and sep- 
taria), Jurassic fossils (Ostraea, Gryphzea, and five 
species of Ammonite), are sparsely distributed in frag- 
ments through the ‘‘till.’’ Details are reserved for 
the B.A. Committee on “ Erratic Blocks.” 
No trace of any crystalline rock (Scandinavian or 
otherwise) has been seen. 
Referring to ‘‘Geology of Oxford and the Valley 
of the Thames,” by the late Prof. John Phillips, 
F.R.S. (p. 461), one sees that the ‘‘northern drift” 
column receives ample confirmation from the facts 
stated above. 
Taking into account the topography, it would 
appear that both the Harlow drift and the drift of the 
Upper Stort Valley have reached their present latitude 
through the ‘““Elsenham Gap” (B.A. Report, 1910, 
p- 616), and it may perhaps be fairly inferred from 
all the facts to hand that the ‘“‘till-like ’? Boulder Clay 
has been composed of material brought thus far south 
by a tongue of the inland ice of the Chalky Boulder 
Clay stage, while the drift deposits of the Upper 
Stort Valley represent in the main the later work of 
floating ice. A. IRVING. 
Bishop’s Stortford, June 14. 
Campaign against Rats. 
I BELIEVE that it is now unanimously admitted 
that the rat, both black and brown, is an unmitigated 
nuisance, both on account of the damage these 
rodents do and also because of the danger of plague 
and other diseases being spread by them. 
The Sheffield and District Working Terrier Asso- 
ciation has for the last two years been doing its best 
to lighten the scourge in this district; but, of course, 
isolated effort is useless. Why should not ratting 
clubs be formed in various parts of the country to try 
