THALAMIFLORiE 129 



Two groups have been made in this order accord- 

 ing to the character of the calyx. 



In the group Silenoideas, the flowers are gamo- 

 sepalous and hypogynous. The sepals are united into 

 a tubular or bell-shaped calyx. The group includes 

 Silette, Lychnis, Gypsophila (not British), and Dianthus. 

 As a type of this group Ragged Robin is described in 

 this volume. 



The other group, Alsinoideae, comprises those 

 members of this order in which the flowers are poly- 

 sepalous, and the stamens are frequently perigynous. 



The sepals are free, or only very slightly connected 

 at the base. The Greater Stitchwort belongs to this 

 group, and Field Chickweed — the latter also described 

 in this volume. 



By some authorities a third group is made to 

 include Polycarpon, with membranous stipules, as 

 in Spergula and Spergularia. Others unite here the 

 Paronychiaceae, including Corrigiola, Paronychia^ 

 Illecebrum, Herniaria, ScleranthuSy and two other 

 foreign types, in which the fruit is an achene or nut. 

 They are included in the Illecebraceae by Bentham 

 and Hooker, whose system is largely followed in this 

 country. 



There are about thirteen hundred species and sixty 

 genera of plants of this order, which are world-wide 

 in range. The British types are enumerated in the 

 Introductory Volume. 



As a whole, they are herbaceous, or in some cases 

 undershrubs. The leaves are mainly opposite, entire, 



9 



