OEGAUOGEAPHY. 51 



AxiUary when on the point where the veins fork (131). 

 Compital when on the angular crossings or points of confluence of 

 two or more venules or veinlets. 



MedM when in none of the above positions, but some intermediate 

 part of the veins or the branches (21, 28)., 



They are superficial, or immersed in the substance of the frond, 

 or elevated above its surface, and then columnar (plate I. fig. 9) or 

 globose. 



In form they are— 

 Punctiform when small and dot-like. 

 Elongated when long and liue-like. 

 Amorphous when of no defined form (46). 



2. Sporangium,. 



The spore-cases, or sporangia, are the organs which contain the 

 reproductive spores, and are borne in masses upon the receptacles. 

 They are thin and transparent, or horny and opaque, unilocular 

 and globose, oval or pyriform, usually pedicellate, which is articulate, 

 but sometimes sessile, and either furnished with a more or less 

 complete articulated elastic ring (annulate) (plate I. fig. 1), or 

 destitute of a ring (exannulate) (plate I. fig. 4). In annulate 

 sporangia the ring is said to be 



Vertical when it rises immediately from the apex of the pedicel (of 

 which is a continuation), and passes vertically over the apex 

 of the sporangium (plate I. fig. 1). 



Horiwntal when it passes horizontally round the sporangium 

 either at or about its middle (plate I. fig. 2), or at the apex 

 {apical) (plate I. fig. 3.) 



Oblique when it has neither of the above directions, but passes 



round the sporangium in some direction intermediate 



between them. 



When the sporangia arrive at maturity and are under certain 



favourable conditions as to dryness, the elasticity of the ring causes 



them to burst open with force and sound sufficient to be heard, and 



this takes place in a direction at or very near to a right angle with 



the direction of the ring. In exannulate sporangia the opening 



takes place by a simple slit or pore (plate I. figs. 4 and 5). 



E 2 



