HEP A TICM. 



^51 



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. 242. — Female receptacle of Marchatitia J>oly- 

 morpha seen laterally from below ; st stalk with two 

 channels ; sr the radiate outgrowths of the disc ; fc the 

 intermediate perichistium ; /sporogonia (X about 6). 



Fig. ■z^'i.—Marchantia 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 Meizgeria, have been represented in detail in Fig. no ; in jineura 

 and Fossombronia 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 



