SUM^fARIES OF EAST RIDING PLANTS. 43 



From the physiographical account given above, it will 

 be evident that a flora has not always existed in the East 

 Riding. For example, this would be the case when most, 

 or all of the area was still under ice during the Glacial Epoch. 

 If vegetation existed then, it must have been of a ver}- 

 meagre character ; indeed, such as obtains in ice-covered 

 ■Greenland to-day. Nor is it easy to imagine that the plants 

 now growing here have all arrived on the soil at one and 

 the same time. On the contrary, there is every evidence 

 to show that plants, like other organisms, obey the laws 

 of locomotion and migration which enable them to secure 

 ■congenial soil and climate. And these laws are not 

 manifested by leaps and bounds, but are comparatively slow 

 iind gradual processes. Some. of the plants of our ballast- 

 hills and dock-side "wastes" and "tips" arrived only 

 yesterday, and were welcomed by none save the botanist. 

 Some of our old garden plants introduced for cultivation 

 may be a century or two old, and some of their descendants 

 are now wild and thoroughly well established denizens ; 

 •others came doubtless with seed corn, and look almost 

 like aborigines, so completely have they made themselves 

 at home in our fields ; whilst others again have been here 

 since shortly after the Ice Age just spoken of, and by so 

 long possession of the soil may be reckoned truly natives. 

 Some plants once with us, or said to have been with us 

 once, are not now known, and in the Flora are reckoned 

 " incognita." 



Adopting the late Henry C. Watson's designations for 

 these groups, we have as follows : — 



1. Natives, or aboriginal possessors of the soil. 



'2. Colonists, or introductions of the historic periotl. 



3. Denizens, or ■\vell-establislied horticultural plants. 



4. Aliens, introduced plants not fully established. 



5. Incognita — not now known to exist, or requiring conlirniation. 



Summarised approximatel)^ the 1035 species recorded in 

 the Flora give the following totals : — 



For Britain. East Riding. 



Natives 935 764 



Colonists and Denizens 100 114 



Aliens 1.35 137 



Incognita 30 20 



1-200 1035 



