368 The British Association at Winnipeg. 
by an increase of shade. This favours those plants which need 
shade for germination, and opposes a check to those requiring light. 
Plant-invasions, the influence (chiefly destructive) of man, topo¬ 
graphic and climatic changes, were also discussed as causes of 
succession. In general it is probable that every case of succession 
is occasioned by the co-operation of a number of causes. 
Professor F. Ramaley (of Colorado), in a paper on the relations 
of the Flora of the Rocky Mountains to Climate, concluded that in 
the main it is temperature, rather than topography, soil or rainfall, 
which permits or restricts the extension of plants over wide areas. 
Professor B. E. Livingston (who has recently been appointed 
to the new Chair of Plant Physiology at the Johns Hopkins 
University of Baltimore) next gave an account of the porous cup 
atmometer as an instrument of ecological research. Dr. Livingston 
laid great stress on the importance of the measurement of evaporation 
in the systematic study of plant environments. “ A properly 
constructed atmometer” (or evaporimeter), said he, “ will automati¬ 
cally sum the various meteorological elements as they influence the 
plant, and may he said approximately to integrate the march of 
plant environment above the soil surface. From the curve of 
evaporation for the growing season in any region, together with the 
rainfall and certain physical data in regard to the soil, the general 
nature of the vegetation, in an ecological sense, may be quite 
clearly deduced.” The author then described the porous clay 
atmometer which he himself devised, and has extensively used. A 
number of useful precautions to be observed in its use were 
suggested. 
Professor R. H. Yapp gave the results of some observations 
and experiments on Spircea Ulmaria. The curious seasonal 
differences found in this species, in respect to the formation of 
glabrous and hairy leaves, had been previously described. It was 
now shown that these differences correspond in a remarkable way 
with the vertical differences (due to stratification of the vegetation) 
and al$o to the annual march, of evaporation and light intensity. 
Attention was also drawn to the marked similarities in the distribution 
of (1) anthocyan in young leaves, (2) hairs on partly hairy leaves of 
Spircea, and (3) withering in leaves in exposed situations. 
Professor Parnmel (of Iowa) in discussing the delayed germin¬ 
ation of seeds, showed that exposure to the rigorous conditions of 
winter resulted in more rapid and perfect germination than housing 
in a dry condition until the spring-time, 
