1726 A TEXTBOOK OF THEORETICAL BOTANY 



which are arranged in two whorls. They are hypogynous or occasionally 

 perigynous, and apparently obdiplostemonous, that is to say the stamens 

 of the outer whorl appear opposite the petals rather than alternating with 

 them. In some genera the stamens may be reduced to five, four or even 

 one. The anthers are two-lobed and dehisce longitudinally. 



The gynoecium consists of from two to five carpels, which are 

 syncarpous, with free styles. The ovary is unilocular and superior; the 

 ovules are usually numerous, campylotropous and arranged upon a central 

 column, i.e., the placentation is apparently free central (but see p. 1229). 

 The carpels as judged by the positions of the styles are sometimes opposite 

 the sepals as in Viscaria and Spergitla and sometimes opposite the petals 

 as in Githago. This has led to the suggestion that in the ancestral complete 

 flower the gynoecium consisted of two whorls, of which one is now absent. 

 In Viscaria it is the antipetalous, while in Githago it is the antisepalous 

 whorl which is absent. 



The fruit is generally a unilocular capsule which dehisces valvately at 

 the apex, the seeds being scattered by the censer mechanism. 



The seeds are endospermic and the embryo is curved or eccentric in 

 the endosperm. The funicle is sometimes unusually conspicuous. 



The family comprises some eighty genera and about 1,300 species 

 which grow mostly in the temperate regions of the northern and southern 

 hemispheres. A few are found in the tropics, mainly on mountain tops. 



The chief distinguishing anatomical feature is that the stomata possess 

 two subsidiary cells placed at right angles to the guard cells. So far as the 

 stem is concerned, the cork is generally internal and there is often a ring of 

 superficial sclerenchyma. Much of the xylem is frequently unlignified. 



Biologically as well as morphologically the family falls into two distinct 

 groups. The more advanced group is the Silenoideae and the less advanced 

 the Alsinoideae. 



I. Alsinoideae 



In this sub-family the flowers are polysepalous and the stamens often 

 perigynous; gynodioecism is common. The petals are small, white and 

 simple. It is divided by Pax into a number of groups as follows: 



{a) Fruit a capsule opening by teeth or valves. 



1. Ahineae. The styles are free to the base and the leaves exstipulate. 



Stellaria, Cerastiiim, Sagina and Arenaria. 



2. Spergiileae. The styles are free to the base but the leaves are stipu- 



late. Spergiila and Spergularia. 



3. Polycarpeae. The styles are joined at the base. Drymaria and 



Polycarpon. 



[h) Fruit an akene or nut. 



1. Paronychieae. The flowers are all developed equally and the leaves 



are stipulate. Corrigiola, Paronychia, lllecebrum and Herniaria. 



2. Dysphanieae. The flowers are all developed equally but the leaves 



are exstipulate and alternate. Dysphania. 



