HEPA TICjE. 
357 
The bilateral structure is distinctly manifested not only by the thalloid forms, which 
mostly cling closely to the substratum, in that the sexual organs are formed only on the 
upper side or the one exposed to the light, and rhizoids and leaves on the under or 
shaded side ; but in the foliose forms also this tendency is clearly shown, whether they 
cling closely to the substratum or rise from it obliquely. This bilateral structure is 
manifested not only in the different mode of the formation of the leaves on the two 
sides, and in the expansion of the ramifications in a single plane, but is also deter- 
mined, both in the foliose and in the thalloid forms, by the growth of the apical region 
of the shoot. Even the youngest segments of the apical cell exhibit it, as is shown 
FIG. ■z^2—Marcha7itia polymorpha ; A vertical section through a female receptacle hu ; bb leaves ; h root-hairs in its 
channel ; g large cells between the air-cavities of the upper side ; B horizontal section of half an older receptacle and of its 
stalk St; chl the chlorophyll-bearing tissue of the disc, below which are large hyaline cells ; a unfertilised archegonia: // peri- 
gynia of fertilised archegonia ; C vertical longitudinal section through the receptacle ; a two archegonia ; pc perichcetium [pc 
in Fig. 242). 
in the different organisation of the upper and under sides, and in the similarity (though 
not symmetrical) of the right and left sides of the shoot. 
Enough has already been said on the position of the apical region in an anterior 
depression in the thalloid forms, as well as on the termination of the filiform stem 
in the leaf-bud of the foliose genera. The form of the apical cell, and its segmen- 
tation in the thallus of Metzgeria, have been represented in detail in Fig. 110 ; in yi?2eura 
and Fossombronla it is also two-sided. In Blasia, on the other hand, Leitgeb states 
that it is four-sided, and forms four rows of segments, a dorsal, a ventral, a right, and a 
left row. ' This may be most easily represented by supposing a wedge-shaped apical cell 
