44 
given in thousandths of a millimetre, one of which is represented 
by the Greek letter //. This unit is one twenty-five thousandth 
part of an inch, very nearly. The measurement of the spores is 
best taken by a micrometer attached to the eye-piece of the mi- 
croscope. 
To find the stomata (absent only in I lacustris) is not at all 
difficult when fresh leaves are procured, or even in those which 
have been preserved in alcohol Fragments of the epidermis are 
easily removed, and with a power of 200-300 diameters the stoma- 
ta when present are easily distinguished When apparantly ab- 
sent the tip of the leaf should be carefully examined, as in some 
species notably in I Tuckermani, and in some forms of J. echinos- 
pora, they are absent elsewhere. In the case of dried specimens, 
short pieces of the leaf may be first soaked, and then boiled pre- 
ferably in a weak alkaline solution. Portions of the epidermis, can 
then be scraped off and examined ; or if the manipulator is suffi- 
ciently skillful, the section may be opened by a slit, and the whole 
internal portion removed, leaving the epidermis in good condition 
for examination. If the leaves have become dark brown and part- 
ly decomposed, their examination requires considerable patience. 
In searching for bast-bundles in the leaves, it is necessary 
to make the thinnest possible transverse sections. Elder pith, 
which can be found at most country roadsides, or is procurable 
from the watchmaker, furnishes a good and ready means of making 
these thin sections When selecting the elder stalks those of the 
previous year should be chosen as the pith is large and the woody 
portion readily separable A short piece of the pith is split nearly 
to the end, a portion of the leaf is placed lengthwise in the slit, 
and by the use of' a sharp razor and some patience, very thin sec- 
tions may be obtained. These sections may then be transferred 
by a moistened needle to a glass slip, or first floated on water and 
removed by a pipette. A weak solution of caustic soda or potassa 
or even of baking powder is then added, a cover glass placed over 
all and the slip heated over a lamp until the boiling point of the 
