102 THE STORY OF PLANT LIFE 



Channel Islands. In Yorkshire it ascends to an 

 altitude of 1200 feet. 



One knows where to seek for this lowly gem of 

 a flower, for it has a great affection for the wall- 

 top. I have seen it covering the sloping sides of 

 a thatched roof in great profusion. The mud-wall, 

 where also grow Wallflowers, Stonecrops, Shepherd's 

 Purse, Thale Cress, etc., is its favourite haunt, and 

 in some parts these primitive forms of enclosure 

 still exist, but unfortunately for the botanist (and the 

 artist's sense of the picturesque), the top itself is now 

 being replaced by slates or cement, thus extermi- 

 nating the mural vegetation. The same applies to 

 thatched roofs, which are being replaced by tiles or 

 galvanised roofing. But in these days of vandalism 

 and jerry-building what else can one expect ? 

 Another favourite station for Whitlow Grass is a 

 mole-heap, which also the cryptogamist will find a 

 profitable hunting-ground for mosses, and on heathy 

 sandy soil many ericetal lichens. Other habitats 

 are the loose soil about railway-sidings, banks, and 

 paths in gardens, or by the wayside. 



This plant is one of those that may be regarded in 

 the aggregate sense as a species with many forms, 

 in the Benthamian sense, as is done by the 

 " Lumpers," or, in the Jordanian sense, as a number 

 of small species in the making, differing in the 

 characters of the siliculas, as is done by the 

 " Splitters." The modern tendency is to divide up 

 these aggregates, owing to the theory of mutation 



