II MUSCINE.E— HEPATIC^— MARCHANTIACEA£ 27 



compact, but in the older parts a modification is observable 

 both on the dorsal and ventral surfaces. In the former, a short 

 distance from the growing point, the superficial cells project 

 in a papillate manner above the surface. This causes little 

 depressions or pits to be formed between the adjacent cells 

 (Fig. 3, C). The subsequent divisions in the papillae are 

 all transverse, and this transforms each papillate surface cell 

 into a row of cells which, as it elongates, causes the pits 

 between it and the adjacent ones to become deep but narrow 

 air-channels, so that in the older parts of the thallus the upper 

 portion is composed of closely-set vertical rows of chlorophyll- 

 bearing cells separated by narrow clefts opening at the surface. 

 In Riccia glaiica, as well as other species, the uppermost cell of 

 each row often enlarges very much, and with its fellows in the 

 other rows constitutes the epidermis. According to Leitgeb's 

 researches this epidermal cell is formed by the first division in 

 the outer cell of the segment, and either undergoes no further 

 division, or by dividing once by a transverse wall forms a two- 

 layered epidermis {R. Bischoffii). On the ventral side the outer 

 cells of the segments project in much the same way, but they 

 remain in close contact laterally with the neighbouring cells, so 

 that instead of forming isolated rows of cells, transverse plates 

 or lamellae, occupying the median part of the lower surface of 

 the thallus, are formed. These remain but one cell thick, and 

 grow very rapidly, and bend up so as to completely protect the 

 growing point. With the rapid widening of the thallus in the 

 older parts these scales are torn asunder, and the two halves 

 being forced apart constitute the two rows of ventral scales 

 found in the older parts. Later these scales dry up and are 

 often scarcely to be detected except close to the growing point. 

 In the case of Ricciocarpus nutans} instead of a single scale 

 being formed, each cell of the horizontal row, which ordinarily 

 gives rise to a single scale, grows out independently, much as do 

 the dorsal surface cells in the other species, and the result is a 

 horizontal series of narrow scales, each one corresponding to a 

 single cell of the original row. These later are displaced by 

 the subsequent growth of the thallus, and their arrangement in 

 transverse series can only be seen in the younger parts. The 

 very rapid increase in length of the dorsal rows of cells as they 

 recede from the growing point soon causes them to overarch 



1 Leitgeb (7), vol. iv. p. 29. 



