KEYS TO THE FERNS OF BORNEO 
423 
A leaf which produces spores is sometimes disting¬ 
uished from other leaves as a sporophyll. In most ferns, 
the sporophylls are like the other leaves or all of the 
leaves of an adult fern may be spore-producing; or the 
sporophyll may differ from other leaves in being smaller, 
or more contracted, or less divided, or of different form. 
In such cases, the fronds are said to be dimorphous. In 
some cases, a part of the frond, almost always the upper 
part, is specialized for spore-production and differs in 
appearance from the rest of the frond. 
The individual spores are almost invisible to the 
naked eye. They are produced in spore cases called 
sporangia. In what are known as the Eusporangiate 
ferns, comprising only two small families, the sporangia 
arise from a group of cells below the surface of the leaf. 
At maturity, the sporangium protrudes, but is not born 
on a stalk, and the cover is several layers of cell in thick¬ 
ness. These sporangia are rather massive structures, 
sometimes more than a millimeter in diameter. The 
overwhelming majority of ferns, constituting the group of 
Leptosporangiatae, have the sporangia formed from a 
single epidermal cell. At maturity, the sporangium is 
born on a stalk, and its wall is a single cell in thickness. 
Running around the sporangium in these ferns, or part¬ 
way around it, is a specialized ring or group of cells 
called the annulus. These sporangia are small, but not 
nearly small enough to be invisible to the naked eye. 
Their structure, however, cannot be seen without a 
microscope. 
The sporangia are sometimes scattered all over the 
dorsal or under side of the leaf; and the fructification is 
then said to be Acrostichoid. In other cases, the 
