DECEMBER. 
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the veins are generally straight and pinnate, the venules uniting 
with the opposite ones at an angle; and the chief differences it 
presents, are, in the regularity of the angles. What we here call 
combined venation, sometimes occurs in the form called transverse- 
marginal, where the oblique, simple or forked veins are united at their 
apices by a marginal longitudinal vein ; sometimes in what we may 
call transverse-costal, where a vein from the costa turns soon after 
emerging from it, and runs parallel with it till it joins the next vein, 
which does the same, and so on, the venules extending free to the 
margin. Among the reticulated class, the term arcuately-anasto¬ 
mosing is used when the opposite venules from adjacent pinnate 
veins meet to form an arc ; compound-reticulated is applied to those 
cases where the veins, venules, veinlets, &c,, form several series of 
areoles, one within the other, and having in the ultimate areoles free 
divaricate ultimate veinlets ; uniform-reticulated is the term used when 
all the veins of a reticulated frond (either with or without a costa) 
are nearly uniform in substance as well as similar in their mode of 
arrangement throughout, including, however, those cases in which there 
is a gradual falling off in substance and in the size of the areoles as 
they recede from the costa, but without any sudden or marked tran¬ 
sition ; and we further propose to distinguish as convergent-reticulated, 
those in which the central areoles are elongated, with two or three con¬ 
verging included veinlets, the external areoles being smaller, and the 
costal ones a single sterile series of the transverse costal form; and as 
semi-reticulated, those in which the costal areoles are obliquely elon¬ 
gated, the veins being there parallel and simple or forked, while near 
the margin they are reticulated in short unequal areoles. The other 
forms of reticulated venation are too numerous and complicated to be 
distinguished by special names. 
A few other terms applied to the venation remain to be noticed. The 
areoles are the spaces enclosed by the veins or their branches. When 
veins are distinct or superficial, they are said to be evident, or less 
accurately external ; when sunk in the substance of the frond, so as to 
become indistinct, they are hnmersed or internal. Moreover, the vena¬ 
tion of every part may be equal, when produced alike on each side a 
midrib or costa; excentric when produced from one side only of a marginal 
or submarginal rib ; and radiate or flabellate when there is no main rib, 
but the veins spread and branch out from a common point at the base. 
In the greater part of the group of Ferns now before us, the sori are 
placed on some particular and definite point of the venation, and the part 
of the vein, or venule, or veinlet, to which the sorus, is fixed, is called 
the Receptacle. We attach some importance to this part, on account 
of its being so intimately connected with the fructification; and it does, 
in practice, prove a useful aid in distinguishing such genera as Pteris, 
Cheilanthes, and Platyloma, in which, while the sori often appear very 
similar, the receptacles are not at all alike. The Sori are the groups, 
clusters, lines, or aggregations of spore-cases. They are generally 
situated on the plane of the frond, or slightly elevated, and are then 
called superficial ; but sometimes they are seated in a groove or channel, 
and are then said to be immersed. The principal forms assumed by 
the sorus are these:— puncUform, i. e., forming round dot-like clusters; 
