APRIL 21, 1923] 
material available in the special hospital and in the 
600 additional beds of the adjacent Medical College 
group of hospitals, 
HUMAN pathology has naturally had first claim 
upon the services of the investigator of 
disease, but a study of plant diseases is probably 
equally essential to human progress, and the timely 
review in Science Progress (No. 67, January 1923), 
by Dr. E. J. Butler, director of the Imperial Bureau 
of Mycology, bears eloquent witness to the great 
activity with which the special problems of plant 
_ pathology are now being attacked. It was only 
towards the close of the last century that the pro- 
pagation of disease in plants was shown to be effected 
in some cases by a filterable virus, but since then 
facts and theories as to virus transmission have 
followed in rapid succession from various Continental 
and American laboratories. Very few observations 
have so far come from British laboratories, and it 
may be hoped that the very comprehensive and 
critical review presented by Dr. Butler will direct 
more attention to this fascinating field of work. 
Many obscure conditions prevailing among growing 
_ plants should receive elucidation as a result of investi- 
_ gation into this problem, while the facilities the 
plant provides for experimental work may enable 
the whole mechanism of transmission by a virus to 
be submitted to a very critical analysis. For more 
than a century it has been known that in certain 
cases of variegation, if a branch bearing variegated 
green and white foliage be grafted upon a plant of 
the same species with normal green foliage, the 
variegated habit will slowly extend to the branches 
formerly bearing normal green leaves. This type 
of “infectious chlorosis ’’ is still of obscure origin, 
and in this case, as with the curious “‘ peach yellows,” 
investigated in the United States, and in the “ spike ”’ 
disease of the sandalwood tree in India, grafting 
appears to be the only artificial method of trans- 
mission. All these puzzling abnormalities, varying 
from innocuous variegation to serious diseases such 
as the “‘ spike ’’ disease, which threaten to extinguish 
a profitable crop, may receive elucidation through 
the study of virus diseases more amenable to ex- 
perimental treatment. 
Among the diseases suitable for investigation, 
perhaps the best known are the “‘ mosaic ’’ diseases, 
so called from the patchy discoloration they usually 
produce upon the plant surface. Tobacco mosaic 
ces a remarkable case of transmission by a 
ighly infectious virus which has been very thoroughly 
examined by H. A. Allard in the United States. In 
this case, if the hairs upon an infected plant are 
carefully cut with a sterile scissors, infection may 
follow if the hairs upon a healthy plant are then 
cut with the contaminated scissors. Originally 
considerable support was given to a theory that the 
infectious principle in tobacco mosaic was enzymic 
in nature, but Allard showed that, although ultra- 
microscopic, the infectious substance could be 
removed from the expressed plant juice by filters 
that left the oxidase activity of the juice practically 
unimpaired. However, the strongest argument in 
favour of an organism is furnished by dilution 
experiments in which the expressed juice, diluted to 
I in 10,000, still retains infectious properties. One 
of the most puzzling properties of the tobacco virus 
is its extraordinary stability to chemical reagents 
usually very toxic to living protoplasm and its 
resistance to relatively high temperatures. In the 
absence of any information as to the life-history 
NO. 2790, VOL. II1] 




NATURE 
55! 
The new Institution is evidently destined to take 

a leading place in scientific medical research and 
teaching in the British Empire. 
Virus Diseases of Plants. 
of the invisible parasite it is impossible to correlate 
this resistance with any special growth form. 
The invisibility of the organism sets an upper 
limit to its size in accordance with the resolving 
powers of the microscope ; experiments with bacterial 
filters, in view of their tendency to clog, do not 
permit a lower limit of size to be assigned with 
confidence, while, on the other hand, the way in 
which a mycetozoan plasmodium will filter through 
a cotton-wool plug, cleaning itself from ingested 
food particles in the process, suggests caution in 
considering passage through a filter a proof that the 
natural diameter of the organism is smaller than 
that of the pore of the filter. 
Although a filterable virus was first demonstrated 
as a cause of disease in the case of the tobacco 
mosaic, plant pathology is not so far advanced in 
its study of the organism as human pathology. 
One great difficulty is that the culture of the 
organism outside the plant has so far proved im- 
possible ; in this respect these are as confirmed patho- 
gens as the well-known group of rust fungi. Some of 
the virus diseases, as potato leaf-roll, net necrosis 
of the tuber, etc., seem to propagate only within a 
special tissue, the phloem. This is worthy of con- 
sideration when attempts are made to cultivate the 
organism on artificial media, as the phloem is relatively 
alkaline in reaction and both cell walls and contents 
are probably very distinctive in chemical composi- 
tion. 
Many of these virus diseases are propagated by 
insects, and Dr. Butler discusses critically the 
evidence which has been brought forward to explain 
the greater success of transmission when the plant 
cuticle is pierced by the insect rather than by needle 
or knife. One interesting possibility is the need for 
a necessary part of the life cycle of the pathogen 
to be completed in the insect carrier, but more work 
is also required upon the natural healing of punctures 
caused by insects and by instruments. The manner 
in which some aphids are also alleged to puncture 
always in the neighbourhood of the phlem also 
provides a very interesting problem for further 
observation and experiment. 
One interesting result of this work is the consider- 
able significance it gives to the aphis as a carrier of 
plant diseases. At the International Potato Con- 
ference held under the auspices of the Royal Horti- 
cultural Society in November 1921, Mr. A. D. Cotton 
pointed out how the recent work of Quanjer in 
Holland and Schultz and Folsom in the States 
emphasised the importance of the relative intensity 
of aphides and possibly other insects in the propaga- 
tion of leaf-roll. This disease, which is of very 
great economic importance, seems to spread from 
plant to plant chiefly in districts where the aphis- 
attack is general early in the season. As a result, 
the disease is transmitted very extensively in the 
warmer English counties, while in the Northern 
Scottish counties its spread may be little or nil, 
coincident apparently with the relative absence or 
late development of aphis infestation. This is very 
suggestive in relation to the proved value of Scotch 
seed-potatoes, and this important problem alone, 
with the new light it throws upon the principles to 
follow in seed-selection, would justify the extensive 
exploitation of this comparatively new field of 
scientific investigation. 
