SPHAGNUM MARCHANTIA. 233 



:and need this mechanical apparatus in order not to collapse \ nor 

 be crushed. The green cells of the leaf expansion are all joined 

 together, and form a network with elegantly sinuous walls, each 

 mesh being occupied by one of the dead cells. The green cells 

 serve for the assimilation of carbonic acid gas ; the empty cells 

 serve, just as do the corresponding cells of the outer cortex of the 

 stem, as a capillary apparatus for the supply of water. Careful 

 observation shows that the number of pores diminishes towards 

 the edge of the leaf, that they are more prevalent on the under 

 side of the leaf, and occupy the sides of the bulging cell- walls. 

 The edge of the leaf is composed of the narrow green cells, and 

 adjoining these of a single-rowed "seam" of narrower collapsed 

 elements containing water, and slightly thickened on the outer 

 surface. Only the end surfaces of these elements appear to be 

 thickened more strongly, and project outwards proportionally. A 

 midrib is \vanting in the leaves, just as is a conducting bundle in 

 the stem ; the plants are in this respect, therefore, much more 

 simply constructed than Mniuin ; more complex on the other 

 hand in the formation of a special capillary apparatus. Cross- 

 sections (Fig. 92 B) give us further information as to the relations 

 between the living and dead cells of the leaf, and show us also the 

 pores (I) in the latter very clearly. 



Thallus of Harchantia. The thallus of Marcliantia polymorpha 

 {the common Liverwort), is widely spread upon damp ground, 

 and especially in damp green-houses on the surfaces of the flower- 

 pots, and on walls and soil, and is readily recognisable by its 

 rounded or saucer-like gemmae-cups, and perhaps also by its 

 more rarely produced disk-like or umbrella-like fructifications 

 (receptacles). It shows a fairly complex structure. The absence 

 of cormophytic organisation does not therefore necessarily entail 

 simple anatomical structure. The thallus is leathery; it branches 

 by forking (bifurcation) of its apex, which lies at the base of an 

 apical depression. If the shoot has forked shortly before, the centre 

 of the previous depression is occupied by a lobe of the thallus, 

 on both sides of which the new apical depressions lie. Along the 

 ^centre of each shoot projects, on its ventral, or under side, an indis- 

 tinctly delimited midrib. From this proceed outwards and forwards 

 obliquely-directed striae curving towards the margin of the thallus. 

 At some distance from the apex the thallus is fixed to the substra- 

 tum by delicate rhizoids springing chiefly from its midrib. If we 

 bring the thallus, with its ventral side turned upwards, under a 



