SECTIONAL TRANSACTIONS.— K. 401 



Results. The polymorphism of iSilene vulgaris Garcke and S. man'tlma L. is sliowii 

 to be very great and the combinations of cliaracters very numerous owing to intra- 

 apecilic crossing. A new scheme for the analysis of wild populations is proposed and 

 examples are given. The relationships of the two species are discussed. Controlled 

 intraspecific and interspecific hybridisation has proved the genetical basis of much of 

 the variation found in nature. 



Considerable light has been thrown on the puzzling taxonomy of Centaurea by 

 intensive genetical and field studies. So-called species have been shown, both by 

 analysis and by synthesis, to be either hybrids or of hybrid origin, with C. jacea L. as 

 one parent and C. nigra L. (C ohscura Jord.) or C. nemoralis Jord. as the other. Natural 

 hybrid swarms have been analysed in localities as far apart as Berkshire and Devon- 

 shire. C. scabiosa L. does not cross with other British species in the author's experi- 

 ence, but both this and other investigated species show a considerable range of inter- 

 specific variation. 



By crossing Saxifraga rosacea Moench with S. granulata L., each with 16 gametic 

 chromosomes, a non-segregating tetraploid was obtained with n=32. This has now- 

 bred true to the third generation of over 500 plants. 



Sex and colour variations have been studied in Ranunculus acris L. and R. bulbosus 

 L. From the breeding and cytological investigations on sex there has emerged the 

 conception of a ' time-factor ' invohang the interval between the initiation of the 

 reduction division in anthers and in ovules. 



The hybrid O. intermedium Willd. has been shown to segregate on selling. On back- 

 crossing with either parent {G. rivale L. or G. urbanum L.) the progeny are nearer the 

 parent used. Back-crosses between G. intermedium and G. rivale can easily be mis- 

 taken for true G. rivale. A wild hybrid swarm is described. 



In Ranunculus ficaria L. and Anthyllis vulneraria L. research on intraspecific 

 variation has made some progress and in Primula vulgaris Huds, the investigation of 

 certain naturally occurring mutations has been completed. 



Conclusions. 1. A conception of ' species ' must include a consideration of isola- 

 lation — geographical (as in Centaurea scabiosa and C. collina), ecological (as in Silene 

 maritima and S. vulgaris), or genetical (as in C. nigra or G. nemoralis and C. scabiosa). 



2. Hybridisation is one of the waj'^s in which taxonomic units are produced (as in 

 Saxifraga potternensis). 



3. Some accepted taxonomic units are not stable but are heterozygous pheno types 

 which segregate on selfing, back-crossing, or further crossing, giving rise to complex 

 hybrid swarms (as in Centaurea plants of the nigra-jacea group). 



4. Species, as commonly accepted, are much more variable, much more complex, 

 and much more heterozygous in intraspecific characters than is generally realised. 



5. The species question is dynamic rather than static, and is to be investigated 

 by methods of experiment with living material combined with field-studies and 

 herbarium and library research. 



Prof. D. H. Campbell. — The Preservation of the Red Wood Forests in the 

 Western Stales. 



Afternoon. 



Visit to Mr. Hiatt Baker's garden at Almoiidsbury. 



Friday, September 5. 



Discussion (Sections K, M) on Mineral Elements in Plant Nutrition. 

 (Opened by Dr. T. Wallace. Prof. W. J. V. Osterhout ; Dr. 

 Gregory ; Dr. Winifred Brenchley ; Mr. G. K. Fraser ; Sir John 

 Russell, F.R.S.). 



Dr. Winifred Brenchley. — The mineral constituents of the plant may be classed 

 as major and minor according to the quantity present. The major elements are chiefly 

 recognised nutrients whose importance varies according to the stage of development 

 of the plant. In barley phosphorus is most essential during the early weeks, and may 

 be entirely withheld later on without detriment to the yield, though the phosphorus 



1930 D D 



