300 FIRST FORMS OF VEGETATION. 
plant is often only a reproductive organ. The com- 
mon mushroom as gathered and brought to table 
is merely the fruit of the fungus. It may be said, 
however, of all fungi that they exhibit less or more 
a well-defined separation into two parts, viz., the 
mycelium or vegetative structure corresponding to 
the thallus of the lichen which is horizontal, and 
the fructification or reproductive structure which 
is vertical; and just as in flowering plants the: 
simple leaf, as is well known, is differentiated 
into bract, calyx, corolla, etc, so among fungi 
all parts are resolvable into modifications of the 
mycelium. 
Owing to this extreme simplicity and unity of 
structure, they possess a remarkable power of re- 
producing and repairing such parts of their sub- 
stance as have been injured. This power, it is 
well known, is always more active as the organi- 
zation of the individual, or the part affected, is less 
complicated ; many of the simplest animals, such 
as the polyps, admitting of being multiplied by 
mere mechanical division almost to an unlimited 
extent. It has been often remarked, that in man 
and the vertebrata generally, the power of regene- 
ration is confined to the replacement of small 
portions of the simplest texture, although in them 
the process of renewal is sometimes very extraor- 
dinary. The more highly organized structures, 
