120 
JOURNAL OF THE PLYMOUTH INSTITUTION. 
A very common British species is Stellaria Holostea, L., some- 
times known as the Greater Stitchwort. Its milk white petals add 
greatly to the beauty of our hedge-banks in early spring, and its 
flowers are sure to be gathered by our children in the making up of 
a nosegay of wild flowers in April and May from their mingling 
admirably in a bunch with Primroses, Wild Hyacinths, and the 
flowers of the Red Campion. The little ones may find these pretty 
flowers all about Plymouth, except on the dry and open limestone 
banks where they are not to be seen at all. By the road between 
Laira Bridge and Elburton, a distance of two miles over limestone, 
the plant occurs in only one or two partially shaded spots, and 
nowhere between Laira Bridge and Plymstock, so far as my 
observation goes. The fact that it does not refuse to grow on 
limestone, if shade and shelter are present, seems to favour the 
idea that it is the dry nature of the rock, and not its composition ; 
in other words its lithological rather than its chemical properties, 
that makes the plant avoid it. The power of a certain kind of rock 
in absorbing, retaining, or parting with moisture would seem to 
have much more to do with determining the character of the 
vegetation on it than has its chemical composition. The Digitalis 
purpurea, L., Common Foxglove, like the Stellaria Holostea, avoids 
limestone, but is quite at home both on the slate and granite. The 
absence from a certain tract of any generally common species 
affords sometimes as interesting and important a fact from a 
scientific point of view as does the presence of a very rare or local 
one. Seldom however, except in quite recent botanical works, are 
such deficiencies noted; indeed the defective knowledge of the 
earlier writers would generally have prevented their making state- 
ments of the kind even if they had themselves known the im- 
portance of the facts. Experience proves that it requires much 
more minute and careful observation to enable one to assert the 
absence of a generally common species from a certain district than 
it does to record the presence in it of a rare or local one. 
The genus Hypericum is locally a very interesting one, from the 
fact that all the eleven species of the British flora occur within 
twelve miles of Plymouth, and that this could not be asserted of 
any other tract of the same extent in the United Kingdom. Every 
variety of soil and situation seems to suit one or another of the 
species. Androscemum belongs to the shaded bank or wooded hill- 
side ; perforatum to the sunny bank ; dubium and tetrapterum to 
