104 
HISTOLOGY OF VEGETABLES. 
FIG. 91 . 
A B D 
has the sealariform character; 
lower down, as shown by c, 
the markings are reticulated ; 
still lower the original spiral 
form re-appears, and to this 
again succeeds a portion of 
ladder - like tissue. When 
stretched, as represented by 
the lower extremities of B 
and c, the sealariform ves- 
sels will unroll either in a 
band, or in single fibres. 
I have on a previous oc- 
casion spoken of an excep- 
tion to a general rule which 
occurs in Ferns, namely, that 
a, a portion of a sealariform 
vessel having nine sides, b, up- 
per portion of a sealariform vessel 
unrolling in the form of a spiral 
band, c, middle portion of the 
same vessel unrolling like an 
ordinary spiral vessel, d, portion 
of a sealariform vessel magnified 
500 diameters, showing the depth 
of the pits. 
in them the membrane is fre- 
quently coloured brown, in- 
dependently of its contents. 
When the colour is deep, the 
vessels collected into bundles, 
and arranged in a peculiar 
form, are easily distinguished by the naked eye. 
