272 Murray and Boodle. — A structural and 
Thus the tips of the pinnae form exceptions to the rule of 
opposite branching which prevails elsewhere in the frond of 
this species, because they give off pinnules on their upper side 
only (Fig, 1 e). The pinnae are constricted at regular in- 
tervals so as to appear to consist of a series of segments 
about twice as long as broad ; separated by transverse walls. 
To prove that the apparent septa were not merely annular 
thickenings, a filament (allowed to swell up in water) was torn 
at one of the constrictions, and the cross wall was seen to 
bulge out under the microscope when the filament was pressed. 
Below each transverse wall of a pinna two (except at the apex) 
opposite pinnules are given off, which lean slightly forward and 
attach themselves to the next pinnae on their respective sides 
(Fig. 1 e). In the lower part of the frond they become con- 
stricted at a varying number of points (dependent on the 
length) and form transverse walls at the points of constriction. 
In the upper part of the frond the pinnules give off short uni- 
cellular branches below some of their septa, thus increasing 
the complexity and compactness of the frond. Owing to the 
comparative shortness of the cells which form the pinnae, the 
pinnules are inserted pretty close to one another, and as they 
are directed slightly forwards (i. e. towards the apex of their 
pinna), they cross one another, so that, when looking at the 
frond, one sees one series of pinnules at the surface partly 
covering another series below. The length of the pinnules is 
about 3-5 times their diameter, their articulations being about 
1 1 times. They are relatively thick, so that in a mature frond 
there are hardly any interstices to be seen, on account of their 
overlapping arrangement. The general arrangement of the 
pinnules in the specimens we have examined is a little different 
from that shown by Harvey \ 
The mode of attachment of the pinnae and pinnules has 
now to be described. When a pinnule has by its growth 
brought its tip into contact with another part of the frond, it 
forms at its apex a special organ of attachment which we 
propose to call a tenaculum (Fig. 1 /). 
1 Phyc. Austr., Tab. 32. 
