PENNY CRESS— continued. 
Though some of the other British forms belonging to the genus Thlaspi are biennial 
or perennial, they are quite local in their occurrence. Of those commonly united 
as T. alpestre Linn6, summer-flowering plants six to ten inches high, with sagittate 
cauline leaves ; flowers an eighth of an inch across and sometimes tinged with pink ; 
and obcordate pods a quarter of an inch long and containing from eight to sixteen 
red-brown seeds, T. sylvestre Jordan, with rounded lobes at the apex of the pod, 
occurs in Teesdale ; at Thornhaugh, Northumberland ; and at altitudes of 2,500 feet 
in Glen Isla and Glen Shee in Forfarshire ; T. occitanum Jordan, with toothed leaves, 
a triangular pod, and longer style, occurs on limestone at Settle in Yorkshire and 
Llanrwst in North Wales ; and T. virens Jordan, with only a slight broad and shallow 
notch to the pod, and an even more projecting style, inhabits the limestone rocks of 
Matlock, Derbyshire. 
T. perfoliatum Linne is a small, spring-flowering annual, about six inches high, 
confined to a small limestone area on the borders of Gloucestershire and Wiltshire, 
with converging pointed auricles to its cauline leaves which thus clasp the stem, but 
are amplexicaul rather than perfoliate ; white flowers only a twelfth of an inch across ; 
and small pods with very short styles and containing from eight to twelve pale seeds. 
T. arvense Linne, the species represented on our Plate, is far more common, 
occurring frequently as a cornfield or roadside weed in the lowlands. It is an annual, 
but grows a foot or more in height, branching but little, and bearing oblong, sagittate 
cauline leaves with toothed margins. From May to July it produces its white 
flowers, which are a quarter of an inch across. Nectar is secreted by four small green 
glands situated on either side of the base of the two shorter stamens ; and, since, 
according to Kerner, the flower is slightly protogynous, insect-pollination doubtless 
occurs ; but, as the anthers of the four long stamens seem to mature and open 
inwards at the same time as, and on a level with, the stigma, self-pollination can also 
occur. 
As is usually the case in the genus, the peduncle elongates after flowering, so 
that the pods, which reach three-quarters of an inch in diameter, are borne in a long 
raceme. They each contain from ten to sixteen dark-coloured, oblong seeds, marked 
with concentric ridges and rows of pits. 
