16 STRUCTURE. 
developed of all the acrogenous plants. In the lower of 
these flowerless orders of plants the whole plant consists 
merely of cellular tissue ; but in the more highly de- 
veloped orders, among which Ferns hold the highest rank, 
both vascular and woody tissues are found. The woody 
matter of the stems of ferns consists chiefly of large 
dotted ducts embedded in hard plates of thick-sided elon- 
gated tissue, which usually assumes an interrupted sinuous 
appearance, but occasionally according to Brown, forms 
a complete tube. The same kinds of tissues are continued 
along the stipes and rachis, where also annular ducts 
occur ; and this vascular system is carried on throughout 
the frond, in the ribs and veins which traverse the 
various leafy parts into which it is divided. 
It is this connection of the veins of the fronds with the 
entire vascular system, which gives to venation its great 
value in the systematical arrangement of these plants. 
These veios are either simple or branched, or dichotom- 
ously forked, with their points free that is, disunited ; 
or their apices are joined together, so as to form what is 
called netted venation. 
From some part of these veins the spore cases arise ; 
and this part, as already mentioned, is called the recep- 
tacle. It occurs sometimes at the apex sometimes be- 
tween the base and apex of the free veins sometimes 
where two or more veins unite, but always at some de- 
terminate point. 
