ON PRIMARY CHARACTERS IN CERASTIUM. 

 By Frederic N. Williams, F.L.S. 



In many important floras Cerastium trigynuin Vill. is transferred 

 to the genus Stellaria on account of the ovary bearing three styles, 

 and when ripe dehiscing by six teeth. The presence of three styles 

 instead of five is therefore in this case regarded as a generic cha- 

 racter. An examination of specimens of many species shows, 

 however, that this is an inconstant character, and is by no means 

 to be relied on for separating Cerastium from Stellaria. The form 

 of the ripe capsule as well as the direction and curvature of the 

 teeth after dehiscence should together be taken as the cardinal 

 character of Cerastium. as distinct from Stellaria: the number of 

 the styles and capsular teeth should not in this connection be taken 

 into account. 



Species of Cerastium may generally be distinguished from those 

 of Stellaria and Arenaria by habit, although not always satisfactorily. 

 At the same time, if the fruit-characters of a typical Cerastium be 

 kept in view, there ought to be no difficulty in defining the genus 

 and in circumscribing the species. As defined and aptly named by 

 Linn^us it is characteristic enough. The form of the ripe capsule 

 in three common species affords a ready illustration. 



In Cerastium, triviale the capsule lengthens considerably after the 

 formation of seeds ; it is then somewhat cylindrical in form, often 

 more than twice as long as the calyx ; in the process it curves like 

 a horn, and dehisces by ten short straight teeth revolute at the 

 margins. The ovary before fertilization of the ovules is ovate- 

 globose, and scarcely longer than the calyx. 



In Stellaria Holostea the ripe capsule is globose, about equal to 

 the calyx in length, and after rupture the six teeth formed by 

 dehiscence extend half down the capsule or beyond. 



In Arenaria serpi/llifolia the ripe capsule is ventricose-ovoid, 

 equalhng or slightly exceeding the calyx, and dehisces by six plane 

 straight teeth not revolute at the margins ; neither is the capsule 

 so deeply cleft as in a Stellaria. In many species of Cerastium the 

 capsule is straight, but the teeth are revolute at the margins as in 

 C, nemorale Bieb., or else at the apex (circinate-convolute) as in 

 C. tomentosum L. 



Whether the capsular teeth are opposite the sepals or alternate 

 with them is a more important character than the fact of the 

 gynoecium being isomerous or meiomerous in relation to the seg- 

 ments of the calyx. There is therefore greater reason for keeping 

 up the genus Malachium than for referring Cerastium trigynum to 

 Stellaria, where it was first placed by Linnaeus. The same character 

 satisfactorily serves to distinguish Spergula from Spergularia. In 

 Spergula arrensis L. the five valves of the capsule are opposite the 

 sepals, and before dehiscence the five styles are alternate with the 

 sepals. ^ In Spergularia rupicola Lebel there are only three styles, 

 but in Spergularia grandis Camb. the five styles are opposite the sepals, 

 and the five valves of the capsule are alternate with the sepals. 



