INTIIODUCTION. 
occurs being so distinctive and so constant that this feature is, by many good botanists, 
emplo3^ed to furnish auxiliary, or even sometimes primary, characteristics for the distinguishing 
of the genera. The principal forms of venation have been distinguished by names, which we 
will endeavour to explain by the aid of references to the figures illustrative of the genera. 
\Yhen the veins are unbrauched, they are said to be simple (Fig. 7, 81, &c.) ; and the same 
applies to venules and veinlets similarly circumstanced. Sometimes thej’ are branched once 
or oftener in a dichotomous manner, and are then said to be forked (Fig. 2, 69, &c.) ; sometimes 
the veins are pinnaielij costceform, that is, resembling a mididb and having the venules or 
branches either simple or forked (Fig. 9, 41, 61, 81, &c.). In some they are equal, that is, 
produced alike on both sides of a central midrib (Fig. 49, 67, &c.) ; in others they are radiate, 
and have no midrib (Fig. 32, 77, &c.); in others they are eccentric, or produced from one side 
of a marginal rib (Fig. 72). In all these cases the apices of the veins may terminate at or 
within the margin of the frond or of its divisions (as seen in Fig. 22, 23, 44, 70, &c.), when 
tliey are said to be direct and free ; but similar forms of venation may occur, having the apices 
of the veins of either the primary, secondary, or tertiary series combined in some way or other. 
If the whole sj^stem of venation is uniformly combined, so as to form a complete iietwork, it is 
then said to be reticulated or netted (Fig. 20, 29, 38, 89, 92, 101, &c.). If the simple, or 
dichotomous, or pinnately costseform veins are united by a continuous vein parallel with and 
close to the margin, this form of venation is called the transverse marginal (Fig. 25, 51, &c.). 
Sometimes the apices of the venules of one series or fascicle combine with the apices of the 
adjoining series, and this is called anastomosing : of this arrangement there are some modifi- 
cations, such as the angular anastomosing (Fig. 10, 61, &c.), and the arcuate anastomosing (Fig., 8, 
11, &c.), which varieties are further modified by producing from their exterior side one or 
more outwardly directed (and called excurrent') tertiary veins or veinlets, which are either free, 
terminating in the areoles or margin (Fig. 10, 12, 59, &c.), or anastomose with the next superior 
venules (Fig. 61, &c.). In some forms of reticulated venation the venules ai’e irregularlj^ com- 
bined, the areoles or meshes producing from their sides certain simple or forked variously 
directed veinlets, which terminate within the unequal-sided areoles. This arrangement of the 
venation is called compound anastomosing (Fig. 14, 31, 57, 71, &c.). The veins are said to be 
external when they are more or less superficial, distinct, or elevated above the surface of the 
frond, and internal when they are immersed in the substance of the fi'ond ; in which latter 
case they often become indistinct, and are to be seen only after careful maceration of the frond. 
In these definitions we have mainly adopted Mr. Smith’s terms and explanations. 
From some part of the venules or veinlets, which part becomes thickened, the fructification 
is produced on what is called the receptacle, which organ is said to be terminal rvhen it is 
situate at the apex of the vein or venule (Fig. 11); medial when situate between the base and 
the apex (Fig. 70) ; axillary when on the point of forking (Fig. 83) ; and compital when 
seated on the angular crossing or points of confluence of two or more venules or veinlets (Fig. 14). 
It consists, in most cases, of one-celled spore-cases or sporangia, more or less completel}^ girt by 
an elastic ring, but in some cases, of sporangia which are many-celled or destitute of the ring. 
These spore-cases are mostly collected into masses, which masses usually consist of nmltitudes 
of crowded spore-cases, and are the sori. The latter are either round (Fig. 14, 57, 67, &c.) 
or elongated; sometimes amorphous, that is of no determinable form (Fig. 23, 29, 30, &c.) 
In most cases they are dorsal, that is situate on some part of the back of the frond ; but some- 
times they are marginal (Fig. 77, &c.), or extra-marginal (Fig. 75, 89, &c.). The elongated sori 
are oblong (Fig. 1, 50, &c.), or linear (Fig. 47, 49, 51, &c.), or continuous (Fig. 38, 71, &c.) ; 
