10 
under-surface, with white or yellow meal. The 
rachises and stipes may also be covered with 
hairs and scales. 
Venation. The venation or the mode of 
arrangement of the veins in the fronds often 
form an important criterion for distinguishing 
ferns. [Plate V.] The midrib of a simple 
frond or of the pinne or pinnules of acompound 
frond is called the costa. The first branches 
of the midrib are called veins and the branches 
proceeding from the veins, veimlets. Veins or 
veinlets are spoken of as free when they are 
unconnected with the neighbouring ones ; anas- 
tomosing when the veinlets of one vein are con- 
nected with those of the next; reticulate when 
they anastomose so as to forma _ net-work. 
Free veins are either simple or branched. 
Branched veins may be either (a) forked when 
they break up into two or more branches after 
leaving the costa ;' (6) #imnate when they run 
from the costa to the margin giving off branch- 
es on both sides; or (c) radiate when the veins 
spread out from a definite point at the base of 
the frond. Areoles or meshes are spaces formed 
by the anastomosing of veins. Veins or veinlets 
are said-to be excurrent when directed towards 
the margin and recurrent when directed away 
