Critical Plants noticed on the Excursion. 121 
Glacial Period, there are botanists who see no reason to forbid 
the belief that some such forms have since spread to the mainland. 
Helianthemum chamecistus Miller Gard. Dict., ed. 8, no. 1 
(1768). The tomentose form (H. vulgare var. tomentosum Irvine 
Ill. Handb. Brit. Plants, 690 (1858) ) mentioned by Dr. Ostenfeld (p. 
117 (1912) ) is by far the commonest British form. The hairy form 
(H. vulgare var. virescens Irvine loc. cit.), however, does occur in 
England, as on a Greensand heath, in western Cambridgeshire. I 
have grown the two forms in my garden for three years; and the 
single separating character remains constant. I think that Cistus 
surrejanus L. Sp. Pl. 527 (1753), as figured in Eng. Bot. ed. 1, t. 
2207, is the second of the above forms with an imperfectly developed 
corolla. C. surrejanus was discovered by Edward Du Bois in the 
neighbourhbod of Croydon, Surrey; and Borrer used to find it 
occasionally in the same county. 
Smith’s Cistus helianthemum and C. tomentosus (=Helianthemum 
tomentosum Gray Nat. Arr. ii, 663 (1821) ) refer to two forms of 
Irvine’s var. tomentosum; but whether or not they are good varieties 
is doubtful. In any case, H. tomentosum Gray (I. c.) is simply Cistus 
tomentosus, Smith, Eng. Bot. t. 2208, which is founded on C. 
tomentosus Scopoli Fl. Carn. ed. 2, i, 376, t. 24 (1772). Both 
Smith’s C. helianthemum and his C. tomentosus had leaves “ densely 
downy, white, and hairy beneath.” Hooker (Brit. Fl. 258 (1830) ) 
thought C. tomentosus was not even a well-marked variety of H. 
vulgare, and reduced it to a synonym; and in this he has been 
followed by all British botanists. I should add that Smith’s figure 
of his C. tomentosus has very differently shaped petals and leaves 
from that of Scopoli. 
In the Cambridge British Flora, 1 propose to restrict the term 
sub-variety to forms like the above which are distinguished by a 
single but constant character, i.e., a character which is, so far as one 
can judge, not directly caused by habitat-conditions, as seems to be 
the case with regard to the two forms of this species. To forms 
whose characters appear to be directly due to habitat-conditions (cf. 
Polygonum amphibium forma terrestre), | propose to restrict the 
term forma. Varieties will be those forms which are separated by 
more than one constant character, but which are not sufficiently far 
removed from their allies to be treated as species. 
Stellaria dilleniana Moench Enum. Pl. Hass. 214, t. 6 (1777); 
S. graminea var. BL. Sp. Pl. 422 (1753); S. palustris Retzius Fl. 
Scand, ed. 2, 106 (1795); S. glauca Withering Arr. Brit. Pl. ed. 3, 
K 
