TRANSACTIONS OF SECTION K. 743 



degenerated into heaths or moors. In several limestone districts, howeTer, the 

 author has, in his study of ash woods, observed the following successions of 

 associations : — 



(i) At altitudes below about 1,000 feet — {a) Limestone (or natural) pasture; 



(b) limestone heath; (c) copse of hawthorn {Crateegus), hazel (Coiylus), ash 

 (Fraxinus), &c. ; (d) ash wood. 



(ii) At altitudes above 1,200 feet — («) Limestone pasture; (b) heath pasture; 



(c) Calluna moor. 



(iii) At altitudes below 1,000 feet — («) Limestone sen es ; (ft) copse of 

 hawthorn, &c. ; (c) ash wood. 



(iv) At altitudes above l,2oO feet — («) Limestone screes; (b) limestone 

 heath; (c) Ca//«w« moor; (f7) ^/7o/>^o;-mj» moor at 1,500 feet. 



6. In a small area like England it would appear that the plant associations 

 are determined much more by edaphic than by climatic factors. Of the edaphic 

 factors the occurrence of humus and humous acids is one which is highly 

 important, and deserving of more serious attention than cecologists have yet 

 bestowed upon it. 



7. In this communication a plant formation is regarded as an historical series 

 of plant associations, beginning as an open association, passing through inter- 

 mediate associations, and finally becoming a closed association. 



8. An open association is usually dominated by one plant, and the number 

 of other species is small. An intermediate association either consists of a number 

 of smaller vegetation units (plant societies), as in the case of the dime marsh 

 association, or is dominated by several plants, each of which possesses the same 

 plant form as in the case of the reed swamp. The number of species in an 

 intermediate plant association is often very large. A closed association is again 

 dominated by one plant, and the number of species in the association is small. 

 The ground is not fully occupied by plants in an open association, whereas in 

 a closed association plants cover all the available ground. Intermediate associa- 

 tions pass gradually into each other, but the extremes are easy to differentiate. 



10. Corn Smuts and their Propagation. By Professor T. Johnson, D.Sc. 



The author described the important results obtained by Brefeld in his investi- 

 gations of the mode of propagation of the smuts. 



1. Kiihu's observation of the infectibility of the oat seedling holds good. The 

 fungus reveals itself in the same season by its dark spore masses in the oat 

 inflorescence. 



2. Brefeld shows that in wheat and barley the seedling is not attacked. The 

 smut spores reach healthy plants by the aid of the wind, and gain entrance 

 through the flowers. The fruits of such infected plants appear healthy in the 

 harvest, but in the following season give a smutted crop in artificial culture.*, even 

 to totality of infection. 



3. Thus, while fungicides and I'otation of crops are effective means of protecting 

 oats, fungicides are useless against barley and wheat smut. The seedlings are 

 immune to attack, and the fungicides do not reach the internal mycelium. Safety 

 lies only in securing seed from healthy crops. 



4. The writer suggested the use of the term ' heterositic ' in speaking of the 

 smut fungus, by contrast with the term heteroecious as applied to the rusts. Thus 

 in oat smut the fungus lives saprophytically in a nutritive solution, and repro- 

 duces itself abundantly in the formation of secondary conidia, or ' spoi idia.' Later 

 the fungus lives parasitically in the oat plant, and hei'e reproduces itself by the 

 production of the smut spores or chlamydospores. Each different kind of food- 

 su pply has its associated spore. 



