February 14, 19 18] 



NATURE 



475 



deepened and widened to their present condition during 

 the relatively short epoch of glacially lowered sea-level. 

 The embayed shores, first used by J. D. Dana as a 

 confirmation of Dai win's subsidence-theory, have none 

 of the characters of recently dissected land. Another 

 point firmly brought forward is the unconformity be- 

 tween the reefs and the floor from which they have 

 grown upward. That floor may be seen, for instance, 

 beneath elevated fringing reefs in the New Hebrides. 

 It has, at some epoch, been subject, not to marine 

 planing, but to subaerial denudation. At Havannah 

 harbour in Efate it must have stood above the sea 

 before the corals grew. The joint evidence of the 

 drowned valleys with their mature forms and of the 

 unconformity of the reefs on an old land-surface points 

 very strongly in favour of Darwin's views. Efate and 

 Oa'hu in the Hawaiian Islands furnish instances of 

 oscillatory movements, and some authors have held 

 these to be incompatible with a broad system o^ subsi- 

 dence. Davis justly styles this objection as " the most 

 singular of all." Finally, the inequality of the depths 

 to which drowning has taken place in adjacent regions 

 is a powerful argument against ascribing the submerg- 

 ence to an increase of water in the sea. Davis, 

 with characteristic width of outlook, believes that 

 "some combination of regional subsidence with Glacial 

 changes of sea-level — or with changes of sea-level 

 caused by movements of the sea-bottom — is worthy of 

 careful consideration as being probably nearer the 

 truth than either process taken alone." But his 

 reasoned conclusion is that subsidence has played by 

 far the greater part. 



In a still more recent paper Davis deals with the 

 Queensland platform ("The Great Barrier Reef of 

 .\ustralia," Amer. Journ. Sci.. vol. xliv., p. 339, 1917)1 

 which he believes to be due in large measure to coral- 

 reef agencies, which produced a mature reef-plain before 

 the subsidence occurred that gave rise to the present 

 barrier reef and the embayment of the coast. 



Grenville a. J. Cole. 



A BACTERIAL DISEASE OF CITRUS. 



DR. ETHEL DOIDGE, mycologist to the 

 Department of Agriculture of the Union 

 of South Africa, who is becoming well known 

 for her researches into the bacterial diseases 

 of plants, is to be congratulated on the excel- 

 lent piece of work which is described in detail in an 

 article on "A Bacterial Spot of Citrus."' At a time 

 when research in phytopathology is largely at a stand- 

 still, it is refreshing to read of such ably conducted 

 scientific investigations in our Colonies as these are. 



The citrus "spot" is a disease of economic import- 

 ance in the citrus orchards of the Western Province of 

 the Cape, and attacks not only the fruit, but also 

 the leaf and the branch of the tree. The fruit is dis- 

 figured and ultimately destroyed, while the attacks on 

 the tissues of the stem cause very commonly gum- 

 mosis in the spring. 



The cause of the disease was ascertained to be a 

 species of Bacillus new to science, B. citrimaculans. 

 A comparative table is given of the characters of this 

 and the two organisms known to attack the citrus in 

 America, viz. Bacterium citriputeale and Pseudomonas 

 citri. The description of B. citrimaculans given by the 

 author, together with its full "group number," may 

 be held up as a model to be followed by workers in 

 this field. The opinion is expressed that very 

 probablv the organism is a soil bacillus, which first 

 invaded rotting fruits lying on the ground, and has 

 now taken on a parasitic habit. The organism loses 



1 Annals of Applied Biology, vol. iii., January, 1917, pp. 53-8'. with 

 lo plates. 



NO. 2520, VOL. 100] 



its virulence rather rapidly when cultivated on artificial 

 media. The most frequent method of infection is 

 through wounds, and the author considers the possi- 

 bility of stomatal infection an open question at present. 

 While preventive measures are not discussed, it is 

 pointed out that any improvement in the sanitation of 

 the affected orchards would doubtless prove beneficial. 

 Since it was found that the organism is very sensitive 

 to copper sulphate, it is suggested that spraying with 

 Bordeaux mixture should be triecL E. S. S. 



THE FLORA OF THE SOMME 

 BATTLEFIELD.^ 



THE ground over which the Battle of the Somme 

 was fought in the late summer and autumn of 

 1 1916 rises gradually towards Bapaume, and at the same 

 i time is gently undulating, with some well-marked 

 branching valleys initiating the drainage system of the 

 area. Before the war the land was for the most part 

 under cultivation, but on the highest levels there were 

 large areas of woodland, such as High Wood and Del- 

 ville Wood, now shattered and destroyed. 



During last winter and spring all this country was 

 a dreary waste of mud and water, the shell-holes being 

 so well puddled that the water has remained in them, 

 and even in the height of the summer there were 

 innumerable ponds, more or less permanent, in every 

 direction.^ 



The underlying rock is everywhere chalk with a 

 covering of loam of varying thickness. As a result of 

 the bombardment the old surface soil has been scat- 

 tered and the chalk partially exposed. One effect of 

 the shelling, however, has been to disintegrate the 

 underlying chalk and produce a weathering effect which 

 has been accentuated by the winter rains, snow, and 

 frost. A general mixing of chalk, subsoil, and scat- 

 tered top soil and also a rounding of the sharp edges 

 have taken place, so that instead of the new surface soil 

 being sterile, the shelling and weathering have " culti- 

 vated " the ' land. That this is so is proved by the 

 appearance of the Somme battlefield during the past 

 summer. 



Looking over the devastated country from the 

 Bapaume Road, one saw^ only a vast expanse of weeds 

 of cultivation which so completely covered the ground 

 and dominated the landscape that all appeared to be 

 a level surface. In July poppies predominated, and the 

 sheet of colour, as far as the eye could see, was superb ; 

 a blaze of scarlet unbroken by tree or hedgerow. Here 

 and there long stretches of chamomile {Matricaria 

 chamomitla. L.) broke into the prevailing red and 

 monopolised some acres, and large patches of yellow 

 charlock were also conspicuous, but in the general 

 effect no other plants were noticeable, though a closer 

 i inspection revealed the presence of most of the common 

 weeds of cultivation, a list of which is given below. 



Charlock not only occurred in broad patches, but 



was also fairly uniformly distributed, though masked 



bv the taller poppies. Numerous small patches were, 



I however, conspicuous, and these usually marked the 



I more recently dug graves of men buried where they 



! had fallen. No more moving sight can be imagined 



than this great expanse of open country gorgeous in its 



display of colour, dotted over with the half-hidden 



' white crosses of the dead. 



In all the woods where the fighting was most severe 

 I not a tree is left alive, and the trunks which still stand 



I 1 AVrHged from an arliclr by Capt. A. W. Hill, Assistant Director, Royal 



I Botanic Gardens, Kew, in ihc A'ein BuUetinof Miscellaneous Information, 



\ Nos. o and 10, 1917, by permission of the Con'roller ofH.M. Stationery 



1 Offic. 



I 2 For a description of t>ie battlefield shortlv after th»^ fighting Mr. John 



1 Masefield's recently published book, "The Old Front Line " (Wm. Heine- 



I mann), should be rend. 



