68 MOSSES AND FERNS chap. 



already seen, are exactly intermediate in character between the 

 highest Ricciaceae, such as Tessalina, and the lower Marchantiese, 

 and this is true both of the structure of the thallus and the 

 sporogonium. Corsinia differs from all the higher Marchantieae 

 in the character of the ventral scales, which are formed in more 

 than two rows, like those of Ricciocarpus . Boschia, the other 

 genus, has two rows of scales of the ordinary form. The 

 archegonia are borne in a group in a depression upon the 

 dorsal surface of the thallus, but are not formed upon a special 

 receptacle, although after fertilisation the cells at the bottom of 

 the cavity multiply actively and form a small prominence upon 

 which the young sporogonia are raised, and this may perhaps 

 be the first indication of the archegonial receptacle in the other 

 forms. 



The TargioniecB include the two genera Targwnia, which 

 has been already described at length, and Cyathodium} a genus 

 whose development is not sufficiently known to make its 

 systematic position quite certain. In the position of the sexual 

 organs, and the formation of the two-valved involucre about 

 the fruit, as well as the position of the latter, it corresponds 

 closely to Targionia, but the structure of the thallus is 

 extraordinarily simple, there being practically but two layers of 

 cells with large irregular air-chambers between. While two 

 sorts of rhizoids are present, those that represent the papillate 

 type of the other Marchantiaceae, while thicker walled than 

 the others, do not develop the projecting prominences. 

 Indeed the whole structure of the plant is curiously reduced, 

 and Leitgeb describes it as resembling the young plants of 

 Marchantia or Preissia. The development of the sexual 

 organs is but imperfectly known, and the suggestion of 

 Leitgeb's, that possibly the antheridium is reduced to a single 

 cell, seems hardly probable in view of the structure of the rest 

 of the plant. The sporogonium has the stalk and foot exceed- 

 ingly rudimentary, but the upper part of the capsule shows a 

 zone of cells whose walls are marked by peculiar ring-shaped 

 thickenings, and opens regularly by a number of teeth, which 

 on account of the thickened bars upon the cell wall offer a 

 superficial resemblance to the peristome of the Bryineae. As 

 in Targionia the archegonia arise near the apex of the ordinary 

 shoots, and no proper receptacle is formed. 



1 Leitgeb (7), vol. vi. p. 136. 



