8 



A FEEN BOOK FOE ETEEYBODX. 



It has been remarked that the fructification of ferns is 

 borne on the back or margin of the fronds, or more rarely 

 occupies the whole surface of special and metamorphosed 

 fertile fronds. It is the fructification which appears in 

 brown dots or lines, or confluent masses, on the under 

 surface of the fronds of our common ferns. The dots or 

 lines, which are only visible as such to the naked eye, 

 when examined by a lens or the low powers of a micro- 

 scope are found to consist of clusters or tufts of brown 

 capsules, which are the theccd or spore-cases. A single 

 capsule is a tlieca^ but a cluster of them as they grow, 

 whether in a circular tuft or in an elongated cluster, is 

 called a sorus (from^oro^, Grreek for "aheap''). The^ori 

 are occasionally naked, but more commonly covered, in 

 the first instance at least, with a membranaceous covering 

 called an indusium (Latin, "a shirt"), or involucre (imo- 

 lucrum — Latin, " a cover "). The presence or absence of 

 this cover, and its form, when present, are of great im- 

 portance in determining the genus and species to which 

 a fern belongs. It may be kidney-shaped, and attached 

 by one side ; or circular, and attached at the middle ; or 

 attached all round, and breaking at the apex when mature, 

 enclose the spore-cases in a kind of cup ; or the over- 

 lapping of the margin of the frond may constitute a kind 

 of cover or indusium. The form is variable, and hereafter 

 it will be seen how this variation, which is permanent as 

 far as the members of a genus is concerned, assists in 

 classification. 



Eeturning to the tJiecce (Greek, " a box ") or spore- 

 cases, sometimes called sporangia (a compound of two 

 Q-reek words signifying "spore-vessel"), we must examine 

 their structure more intimately. Each spore-case is some- 

 what globose, generally mounted upon a short stalk, and 

 girt by an elastic ring '{annulus). This ring may pass over 

 the top of the spore-case (fig. 3), and in that case is ver- 

 tical; or its direction may be oblique (fig. 4). 



In some ferns the spore- cases are entirely destitute of 

 a ring, and are termed exannulate. These generally split 

 down the centre, and, as far as the British species are con- 



