158 SECOND GROUP.— MUSCINEAE. 



group. No capsules are ever found in Anthoceros which can be properly said to have 

 ceased to grow ; in Notothylas this growth lasts comparatively but a short time, and 

 its capsules therefore are shorter. In this respect the genus Notothylas forms a 

 transition to the Jungermannieae, as well as in the structure of its capsule, which, 

 as was said above, either has not the columella which is characteristic of Anthoceros 

 and Dendroceros, or one which is only a secondary differentiation inside the spore- 

 chamber. 



2. Series of the Marchantiaceae. 



The Marchantieae, in the narrower use of the term, and the Riccieae, once looked 

 upon as an independent family, really form one natural group, as Leitgeb has lately 

 shown, the two subdivisions being connected together by intermediate forms supplied 

 by the Corsinieae, that is, by the genera Corsinia and Boscliia. 



a. The Riccieae form a flat thallus with dichotomous branching, rooted to the 

 ground or floating in water ; several apical cells, according to Kny, lie close together 

 in the anterior sinus of the branches, and are segmented by walls inclined upwards and 

 downwards, and multiplied by vertical longitudinal walls ; Leitgeb, on the other hand, 

 assumes here as in similar cases, in Blasia for example, a single four-sided apical cell. 

 The under side of the thallus is occupied by a single longitudinal row of transverse 

 lamellae (they are wanting in R. irysfalHna) formed from transverse rows of ventral 

 superficial cells. At a later period these ventral scales separate into two longitudinal 

 rows, between which are numerous rhizoids with conical thickenings on their inner 

 walls. In Boschia this formation of scales is similar to that of the Marchantieae. The 

 dorsal side of the thallus is formed of a thicker or thinner layer of cells containing 

 chlorophyll, with broader or narrower spaces between them filled with air ; this layer 

 may be called with Lei'^geb the air-chamber layer. In most species of the genus Riccia 

 these chambers run like narrow passages at right angles to the dorsal surface of the 

 thallus, in others, as R. crystalliiia and R. fluitans, they form wide spaces. In the 

 first case they are continued through the epidermis and are only partially closed by its 

 cells being swollen into bladders. In the second case a roof is formed by growth of the 

 surface of the epidermal cells in proportion to the successive enlargements of the air- 

 space, as in R. Jluitans, or where this does not take place, the air-spaces open to the 

 outside along their old breadth, as in R. crystallina. The structure is the same in 

 Riccia natans, in Oxyinitra, Corsinia, Boschia, and many Marchantieae, as in R. 

 fluitans, with the difference only that there is an opening, a stoma, in the roof of every 

 air-chamber; this is present in a rudimentary form indeed in Riccia Jlinta?is, but is 

 often not discernible. The way in which these air-chambers are produced is very 

 peculiar. They are not formed in the tissue by shrinking of the cells from one another, 

 nor by a fissure advancing from without inwards, but they represent depressions in the 

 outer surface caused by certain points in the surface being arched over by the more 

 rapid growth of the neighbouring parts. The cavities thus formed are afterwards 

 covered over by the completion of the growth of the surface in breadth (Fig. 112), but 

 the orifice is as a rule preserved, and represents the stomata of the more highly 

 developed species, the formation of which will be examined more closely when we are 

 describing the Marchantieae. 



The sexual organs also, the antheridia and archegonia, are placed in depressions 

 formed in the same way as the air-cavities. Both are at first papillose outgrowths of 

 young epidermal cells, which as they develope become grown over by the surrounding 

 tissue (Fig. 108) ; this involucre sometimes has a neck which rises high above the 

 sessile antheridium. The archegonia project at the period of fertilisation above the 

 epidermis of the thallus, but are afterwards covered in, and then produce from their 

 oospore the globular sporogonium, which shows those varieties of development in the 

 different genera which have been already pointed out. In Corsinia and Boschia the 



