32 FIRST FORMS OF VEGETATION. 
The leaves of mosses are their most prominent 
parts. To the careless and superficial eye, ac- 
customed to look ata tuft of moss as merely a 
patch of velvety greenness, creeping over an old 
tree or dike, the leaves of all mosses may appear 
precisely similar ; but the attentive observer who 
examines them under a microscope, will find that 
the leaves of different kinds of trees are not more 
distinct from each other than are those of the 
mosses. Indeed, so remarkable and so constant 
is this dissimilarity, that it has formed one of the 
principal bases of their arrangement and classifica- 
tion; and the botanist who has studied them 
thoroughly can identify under the microscope, in 
some cases, the smallest fragment of a leaf, 
although almost invisible to the naked eye. The 
leaves of some mosses are quite plain and pellucid, 
exhibiting no structural arrangement whatever ; 
others are furnished with a nerve which runs 
through the centre and terminates above or below 
the apex ; some are either ribbed and notched like 
a saw on the edge, or quite plain and even; and 
others present the most beautiful and varied net- 
work of cells. Some are linear like miniature pine- 
needles, others ovate and round like the leaves of 
our common deciduous trees. The harmonies of 
colours are beautifully exhibited in their appendi- 
cular parts. The stem, in almost. all the species, 
