HEP A TICM. 



Z^Z 



segmental cells, near to the growing-point of the thallus, divides into an outer and 

 an inner cell ; the outer (superior), which bulges slightly, is the mother-cell of the 

 archegonium and is further divided (as is the case in other Liverworts) by three longi- 

 tudinal walls vertical to the surface of the thallus, into one internal and three external 

 cells. The latter produce the six primary investing cells M^hich, at a later period, 

 give rise to as many rows of cells which are enclosed on all sides by the tissue of 

 the thallus. The internal cell is divided by a transverse septum, into two cells, the 

 lower of which becomes the central-cell, and by division gives rise to the oosphere and 

 the overlying ventral canal-cell ; the upper undergoes transverse divisions in consequence 

 of which a row of neck canal-cells is formed, as also the primary stigmatic cell which 

 subsequently divides cross-wise to form the stigmatic cells. The variations from the 

 mode of formation of the archegonium obtaining in the other Liverworts are thus seen 

 to be slight, and scarcely justify the formation of the Anthocerotese into a distinct class. 

 Fig. 237 C dates from a time at which the above-mentioned details were unknown, but 

 it suffices nevertheless to give some idea of the important points in the process. 



After fertilisation the oospore is divided in the manner described above. Whilst 

 the developing sporogonium is gradually becoming a multicellular body dilated inferiorly 



Fig. "z-^j.—Anthoceros Icevts (after Hofmeister) ; A a branciicd thailus ; .5 longitudinal section of a shoot (X4o); a» 

 antheridia beneath the layer of superficial cells ; C longitudinal section through the apical part of a shoot ; nr rudiments of 

 archegonia {X 500) ; D nr fertilised archegonium in the longitudinal section of a shoot, with an embryo consisting of 

 two cells; E multicellular embryo: A' in .ff a colony oi Nostoc settled in the tissue of the thallus. 



(Fig. 237, E)f the cells of the surrounding tissue of the thallus undergo numerous 

 divisions and form an upwardly projecting involucre which is broken through at a later 

 period by the elongating sporogonium. Differentiation now takes place in the homo- 

 geneous tissue of the sporogonium ; a central cylinder of from twelve to sixteen rows 

 of axially elongated cells is marked out, forming the columella, whilst the cells of the 

 layer immediately adjacent to it undergo division by horizontal walls and give rise to the 

 mother-cells of the spores and elaters ; the external four or five layers of cells form the 

 wall of the future ' pod.' Those cells of the layer investing the columella which are to 

 form elaters undergo one or more vertical divisions. The elaters are here transversely 

 directed rows of cells in which no spiral bands are formed. The mother-cells of the 

 spores round themselves off, become gradually isolated, beginning from the apex of the 

 sporogonium and proceeding basipetally, increase in size, and then divide into four, giving 

 rise to the tetrahedral spores. The sporogonium elongates and becomes a pod-shaped 

 structure of from fifteen to twenty millimetres in height, the brown wall of which, 

 provided with an epidermis bearing stomata, splits from above downwards into two 

 valves. 



