452 SECTIONAL TRANSACTIONS.—K. 
Spheropsis malorum type. Small hyaline micro-pycnospores are also some- 
times produced in the same or separate pycnidia. 
Over one thousand single ascospore cultures of these fungi from various hosts 
have consistently produced one or other of the pycnidial forms mentioned, but 
never both. An active parasitic form of Botryospheria ribis, first found on 
currant and producing serious blight, is only distinguishable at present by its 
parasitism and the production of a bright purplish pink colour on starchy 
media. This parasite, B. ribis forma chromogena, also occurs on Atsculus, 
Rosa, and Pyrus malus, as well as Ribes, as has been demonstrated by cross 
inoculations. 
Both Botryospheria and Physalospora are not infrequently found occurring, 
sometimes together, on the same host, and naturally they have been much 
confused in the literature. 
This study indicates the need of a very thorough investigation of the 
morphology, physiology, and life history of such fungi in order to solve the 
many problems of taxonomy and pathology connected with them. 
20. Prof. W. P. THompson.—Correlation of the Specific Charac- 
teristics in a Cross between a Durwm and a Bread Wheat. 
21. Prof. B. T. Dickson.—The ‘ Black Dot’ Disease of Potato. 
The disease has been under investigation since 1921, and an account will 
Be given from historical, geographical, symptomological, and economic points 
of view. 
The organism isolated from Canadian material, with the saltation thereof, 
is described and compared morphologically and physiologically with Vermicularia 
varians Duco, Colletotrichum tabificum (Hall.p.p.) Pethy., Colletotrichum 
(Hall.p.p.) atramentarium (O. Gara) Taub., and Colletotrichum atrovirens (?). 
Pathogenicity studies (still in progress) are reported. 
22. Mr. C. W. Lowr.—The Freshwater Alg@ of Central Canada. 
22a. Prof. F. J. Lew1s.—Popular Lecture on The Vegetation of the 
Canadian Rockies. 
Tuesday, August 12. 
23. Joint Discussion with Section D on Species and Chromosomes. 
(a) Prof. R. Ruceres Gatss. 
When the ‘ Origin of Species’ was written chromosomes were unknown, and 
until 1900 the history of the chromosomes was worked out quite independently 
of studies in heredity or evolution. Since that time the two fields of experi- 
mental breeding and cytology have become more and more closely interwoven, 
and future advances in genetics will depend upon the closer co-ordination of 
these two fields. The relative fixity in size, shape, and number of the chromo- 
somes in a species is no longer disputed by those who know the facts. The 
changes which must have taken place in the chromosomes from species to species 
are only beginning to be studied, but important evidence of relationships can 
be gained in this way. 
Comparative studies of the chromosome groups in many plants and animals 
lead to the recognition that phylogenetic chromosome changes have occurred in 
a variety of ways. These include polyploidy, or the development of higher 
multiples of chromosome numbers, transverse segmentation or fragmentation of 
certain chromosomes, end-to-end union of certain pairs, crossing of species with 
different numbers, and other changes. 
In tetraploid mutations a new centre of stability arises. Hybrids with the 
parent form have an unbalanced chromosome number. They are largely sterile 
and their offspring revert to the parental types. Hence the condition of tetra- 
ploid mutations is essentially one of physiological isolation. This means the 
beginning of a new line of descent. 
