:o6 



MUSCTNE.E. 



are discoid or shield-shaped sessile or stalked branches that have undergone a peculiar 

 transformation. The archegonia are only in the Targioniese inserted at the apex of an 

 ordinary shoot ; in the other families they are produced on a metamorphosed branch, 

 which rises like a stalk and developes in different ways at its summit. The summit 

 bears the archegonia on its outer or lower side. With the variation in the form of the 

 part which bears the archegonia is connected an equally varied mode of envelopment 

 of the archegonia by involucres. Since it is impossible to describe these structures in 

 a short space, we may take Marchantia polymorpba, the species most perfectly endowed 

 in this respect, as an example. The explanation of the figures (220-222) will suffice 

 to explain at least the most essential points. 



The sporogonium of the Marchantieae, usually 

 shortly stalked, contains elaters which radiate from 

 the bottom towards the circumference {cf. Fig. 215, 

 p. 301). It bursts either at the apex with numerous 

 teeth, or is four-lobed as in the Jungermannieae, or 

 the upper part becomes detached as an operculum. 

 The peculiar gemmae and their cupules have already 

 been described. 



5. The Jungermannieae. In this family oc- 

 cur forms of which the vegetative structure is a 

 true flat leafless thallus, as Metzgeria and Aneura, 

 as well as transitional forms whose flat thalloid stem 

 forms leaves on the under surface (Diplolaena), or 

 whose stem, as in Blasia, elliptical in section in its 

 early stage, becomes broad and leaf-like when older, 

 and produces leaves on both surfaces. Closely allied 

 to Blasia is a genus 'with a less dilated stem, though 

 still always greatly flattened on the upper side, and 

 bearing leaves only above.' The greater number of 

 the genera, however, the foliose Jungermannieae, 

 form a slender filiform stem, with numerous sessile 

 leaves with broad insertions but distinctly diflfer- 

 entiated ; these leaves commonly occurring only in 

 two rows situated on the upper side, as in Radula, 

 some species of Jungermannia, Lejeunia, and Pla- 

 giochila. Normally, however, we find three rows of 

 leaves, one being developed on the under or shaded 

 side (hence termed Amphigastria), the other two 

 rows on the upper side (Frullania, Madotheca, 

 Mastigobryum). In the flagelliform branches the 

 leaves remain very small, and are sometimes almost 

 invisible. 

 The bilateral structure is distinctly manifested in the thalloid forms, which 

 mostly cling closely to the substratum, the sexual organs being formed only on the 

 upper side or the one exposed to the light, rhizoids and leaves on the under or 

 shaded side. In the foliose forms this tendency is also clearly shown, whether they 

 cUng closely to the substratum or rise from it obliquely. This bilateral structure is 

 manifested not only in the diff'erent 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 

 in the diff'erent 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 



Fig. 2C0 bis. — Cell-forms oi Marchniitia foly- 

 rnorpha with thickenings projecting inwards; 

 A an elater (one-half) from the sporogonium, 

 with two spiral bands ; ^'a portion more strongly- 

 magnified ; B a parenchyma-cell from the centre 

 of the thallus, with thickenings projecting in- 

 wards in a reticulate manner ; C a slender root- 

 hair with thickenings projecting inwards, these 

 are arranged on a spiral constriction of the 

 cell-wall ; at Z) a thicker root-hair, with thicker 

 branched projections, and spiral arrangement 

 still more evident. 



