1* 



[Proc. B.N.F.C., 



adjoining ground in these factors of water supply, etc., tended to 

 produce barriers across which a plant might find it difficult to 

 pass. 



In studying the origin of our present flora a time limit might 

 be set to its development, for the Ice Age, whether so utterly 

 destructive of local vegetation as many geologists believed, or 

 whether less severe, admittedly altered conditions of growth so 

 profoundly that the building up of the vegetation of our islands as 

 we now find it may be held to commence with the passing away 

 of that epoch. The only evidence of climatic change since that 

 time, which might have affected vegetation, pointed to a rather 

 milder climate than the present, which prevailed probably about 

 the Neolithic period. Another historical factor of the greatest 

 importance in investigations regarding the origin of our flora was 

 the past distribution of sea and land. There was plenty of 

 evidence to show that the areas of sea which at present separate 

 Ireland from Great Britain and the latter from the Continent are 

 of recent origin — probably more recent than the arrival of the 

 bulk of our flora, thus allowing the migration of plants into our 

 islands across a surface of land. 



Coming to the subject of dispersal, the means by which 

 plants increase by vegetative growth were dealt with first, and 

 then the phenomena of seed dispersal were discussed at some 

 length. It was pointed out that, unlike the young stages of most 

 sedentary organisms of the animal world, the seeds of the higher 

 plants have no powers of locomotion, and rely for their dispersal 

 on external moving agencies — water, wind, and animals. The 

 interesting adaptations by which many species secure wide 

 dispersal by these means were described. It was pointed out 

 that there are definite limits to the possibilities of dispersal by 

 even the most efficient of these means. The fact that only about 

 one plant in ten has seeds which float in water strictly curtails the 

 efficiency of water dispersal. As regards the wind, even very 

 small seeds, if devoid of special parachute devices, fall quickly in 



