NUTRITIVE ORGANS OF CELLULAR PLANTS. -313 



this family an important deviation from their usual 

 state ; they are sometimes disposed in two rows, and 

 instead of embracing the stem at their base, they are 

 prolonged by one of their sides upon it. This is seen in 

 several species of Fissidens, to which, from this appear- 

 ance, the names of pennatum, adianthoides, Sec, are 

 given. These seem, in fact, the leaflets or segments of 

 a pinnate leaf, disposed on both sides of a common 

 petiole, and one would be inclined to believe this, if the 

 extremity of the stem were not often prolonged into a 

 branch or flower. The iUusion goes sometimes even 

 farther, for there happen cases when the neigbouring 

 leaves are partly united together by their sides, and 

 then-, if the stem do not flower, it seems a pinnatifid 

 leaf. We may form an idea of this phenomenon by 

 examining Gymnostomum pennatum, and almost all the 

 species of Fissidens. We will revert to its importance 

 on speaking of the Hepaticae. 



The leaves of Mosses are also sometimes capable of 

 joining together ; they then present two points at their 

 apex, and if they are furnished with nerves, they have 

 two throughout their length. This is accidentally ob- 

 served in Gymnostomum truncatum ; it is possible that 

 it may be an analogous phenomenon which causes, in 

 some species, a double or bifid nerve, as in Neckera 

 Hypyio'ides , Sec. 



Let us observe, lastly, in order to complete what 

 relates to the structure of the nutritive organs of Mosses, 

 that their leaves differ from those of all vascular plants 

 in their cellules, being disposed upon the same plane, 

 so that two distinct layers cannot be distinguished, and 

 they cannot be separated into two parts. This character 

 is still more decided in those HepaticcB which have 

 leaves, and it tends to prove, as I have indicated in the 

 first Section of this Chapter, that what is called a leaf 



