492 SECTIONAL TRANSACTIONS.—K. 
27. Prof. D. THopay.—The Geographical Distribution and Ecology 
of the Genus Passerina. 
_Passerina is a genus of ericoid shrubs endemic to Southern Africa, com- 
prising some fifteen species. The ecoiogy and geographical distribution of the 
species parallel in a striking way their morphological relationships. 
The marked correlation of the species with distinctive habitats, some 
restricted in extent, others more or less continuous over wide areas, would 
vitiate any inference from area to relative age. South Africa is an old land 
surface which has not been submerged since Cretaceous times, and it is highly 
probable that most of the species of Passerina have reached limits to their 
distribution set by climatic and edaphic factors. Willis’ recent suggestion 
that species spread slowly enough on the whole for acclimatisation to keep pace 
with dispersal begs an important question. 
The origin of species in this genus has involved morphological divergence, 
together with a physiological divergence in adaptation to distinctive habitats ; 
a satisfactory theory must account for this. The way in which the species have 
divided the land between them is remarkable. 
28. Prof. R. B. THomson and Dr. H. B. Sirron.—Resin Canals in 
Spruce Wood. 
29. Dr. W. L. Bauus, F.R.S.—Popular Lecture on Cotton. 
Tuesday, September 18. 
30. Joint Discussion with Section M on Virus Diseases of Plants. 
(a) Dr. Patt Murreny.—Virus Diseases of Plants. 
Recent researches in plant pathology have shown that a number of obscure 
maladies are really infectious diseases of the so-called ‘ virus’ type, comparable 
to some of the most serious diseases of man and animals, and not mere conditions 
of ill-health resulting from old age or from unfavourable environmental or 
cultural conditions. 
A brief account of these investigations is given, particularly as regards the 
disease of potatoes long known as ‘ Curl,’ which is analysed and described. 
The traditional control measures are considered in the light of the new con- 
ception of the disease, and a sketch is given of the new avenues of approach to 
more rational and effective methods of combating the disease which are being 
opened. 
(b) Prof. H. M. Quanser.—So-called * Virus Diseases ’ of Plants: 
their Symptoms, Causation, Mode of Dissemination, and Eco- 
nomic Importance from a Physiological Point of View. 
Opinion was formerly divided in ascribing ‘ virus diseases’ to physiological — 
influences, degeneration, and parasites. In the majority of fungoid, bacterial, 
eelworm, and insect diseases attacks are local, but in so-called ‘ virus diseases ” 
these are general. In animal diseases, spreading through blood-vessels is — 
possible. The plant tissue most comparable with blood-vessels is the sieve-tube 
system. Diseases which spread through sieve tubes are more generalised in 
host than are most other diseases. The hypothesis that in ‘ virus diseases’ the — 
phloem is the seat of disturbance is confirmed by :— 
(1) Microscopical evidence in the case of potato leaf-roll, curly-top of beet, bd 
raspberry leaf-curl, &c. ; trite ‘ 
(2) Physiological evidence by prevention of movement of assimilates in the 
same diseases and in some others—e.g. peach yellows and sandal spike. 
(3) Experimental evidence given by grafting experiments where the phloem 
joins first and the symptoms follow the downward sap flow in certain potato 
diseases and mosaic diseases of other plants. 
he 
(4) Biological evidence that disease is spread by aphides sucking the phloem ~ 
