GROUP II. BRYOPHYTA : HEPATIC^). 353 



IQ exceptional cases the autheridia of Anthoceros may be developed at the 

 surface, i.e. exogenously. 



The structure of the adult shoot. The adult shoot, in Anthoceros and 

 Notothylas, consists of several layers of cells in the middle line, thinning 

 out to a single layer of cells at the margins. The tissue in the middle line 

 consists of longitudinally elongated cells, the walls of which, especially in the 

 older parts of the shoot, frequently present reticulate or even spiral thickening. 

 In Dendroceros there is a prominent midrib, on each side of which is attached 

 a laminar portion, consisting of a single layer of cells ; in some species there 

 are no intercellular spaces, but in others there are air-chambers in the mid- 

 rib, which give to its surface the areolated appearance so well-marked in the 

 Marchantiaceae (see p. 338). 



A characteristic structural feature is the presence of apertures of the nature 

 of pores iii the superficial layer of those parts of the shoot which consist of 

 several layers of cells. They occur generally only on the under (ventral) sur- 

 face of the shoot, but in some species of Dendroceros (D. javanicus, crispatus, 

 Breutelii) they are present on the dorsal surface also. The pore is bounded 

 by two guard-cells, formed by the division of one of the superficial cells. It 

 leads into a cavity which is, from the first, filled with mucilage. Into this 

 cavity the Alga Nostoc obtains access through the pore, and there grows and 

 multiplies. The growth of the Nostoc in the cavity is accompanied by active 

 growth of the cells of the surrounding tissue, so that the whole mass projects 

 from the surface of the shoot, and the guard-cells grow and divide, so that the 

 cavity becomes completely closed. At the same time, filamentous septate out- 

 growths arise from the cells of the wall of the cavity, which ramify throughout 

 the mucilaginous mass (comp. Blasia, p. 347). 



The chloroplastids of the Anthocerotaceaa are peculiar, on account of their 

 relatively large size, and of the fact that they occur singly in the cells and con- 

 tain a pyrenoid (see p. 100). 



B. The SPOROPHYTE. The early stages in the development of the sporophyte 

 of the Anthocerotaceae appear to be much the same as in the case of other 

 Liverworts (Fig. 239 C). The oospore divides transversely into an epibasal and 

 a hypobasal half: and each of these divides by two perpendicular walls so that 

 the embryo consists at this stage of eight cells. The cells of the epibasal half 

 divide transversely several times, and then further apical growth in length is 

 arrested by the formation of periclinal walls, marking the differentiation of 

 amphithecium and endothecium, first in the four apical cells, and subsequently 

 in those below them. By the repeated formation of periclinal walls, the amphi- 

 thecium comes to consist of several layers of cells. The hypobasal cells under- 

 go but few divisions, giving rise to a bulbous foot, the superficial cells of which 

 grow out into papillas and penetrate between the cells of the adjacent tissue of 

 the gametophyte. 



The details of the differentiation of the epibasal portion of the embryo vary 

 considerably. In Anthoceros and Dendroceros (Figs. 238, 239) the archesporium 

 is developed from the innermost layer of cells of the amphithecium, a peculi- 

 arity, the only other instance of which, in the Muscineas, is to be found in the 

 Sphagnaceas (p. 331) : the endothecium gives rise to an axial strand of sterile 

 tissue, termed the columella, which is completely invested (except at the base, 



V. S.B. A A 



