STRUCTURE. / 
so that either absolutely or with very slight shelter the 
plants become evergreen ; and those possessing this habit 
are the most valuable for the cultivator where a con- 
tinuous effect is an object. Other species are fragile, and 
of short duration, and produced only during the warmer 
portion of the year, shrinking before the first breath of 
winter : this class, however, contains some of our most 
delicately beautiful species. 
Allusion has been made to the woody fibre, or vas- 
cular tissue which forms the chief substance of the stipes, 
and is continued onwards into the rachis. This vascular 
tissue is carried still further, its ultimate ramifications 
forming the veins which occur in the substance of the 
fronds themselves. Thus the vascular system may be said 
to form the framework of the entire plant, which is filled 
out by cellular expansions. Now it is on some deter- 
minate part of these veins that the fructification is borne, 
and that part, which is more or less thickened, is called 
the receptacle. The vascular system, or venation, 
having thus so close a connection with the production of 
the reproductive organs, its modifications have been very 
properly freely used in some of the modern systems of 
classification. 
The veins are distinguished by different names ac- 
cording to their relative position. The central rib, which 
runs along a simple frond, or a simple portion of a com 
pound frond, is the mid-vein or custa ; its branches are 
ailed veins the branches of these are the venvles ; and the 
branches of these again, when present, are the veinlets ; 
so that whilst v. ins are the first series of branchings from 
the costa, venules are the secondary, and veinlets the 
tertiary series. 
The REPRODUCTIVE ORGANS of ferns consist of spores, 
or germinating atoms, enclosed in spore-cases, sometimes 
called thecce. These spore-cases are mostly furnished with 
a short pedicel, and have extended nearly or quite around 
them an elastic vertical or oblique band or ring (an- 
nulus). the elasticity of which causes them to burst ir- 
