274 Murray and Boodle. — A structural and 
serve the same purpose of attaching branches to one another. 
Agardh refers to the well-known similar structures in several 
of the encrusted Siphoneae ( Udotea , etc.), though he believes 
that among these latter their function may sometimes be that 
of attachment, and may sometimes be connected with the 
deposition of lime. We examined a piece of Microdictyon 
Velleyanum , to see if similar organs were present in that genus, 
and found that an apex of a filament, when it comes into contact 
with another filament, forms crenations round the edge of con- 
tact, which are sometimes sufficiently pronounced to be termed 
rhizoids, but the attachment, which is very firm, must be 
chiefly due to cohesion between the filament-surfaces, which 
is here sufficient without increase of contact-surface by forma- 
tion of long rootlets like those of Struvea. Tenacula occur 
also in Spongocladia j where they resemble those of Struvea . 
In describing species of Struvea , Harvey 1 and Dickie 2 both 
speak of £ anastomosis ’ of filaments, and Harvey further men- 
tions it among the generic characters. This word cannot be 
correctly applied to the frond of Struvea , for though the fila- 
ments become attached to one another by means of tenacula, 
there is no resorption of the double membrane which separates 
the cavities of the cohering filaments. The attachment, how- 
ever, is sometimes very firm, as in .S'. delicatula , described 
below. 
The frond of .S. macrophylla (Fig. 2 a) bears a slight general 
resemblance to that of .S'. plumosa , but differs in the mode of 
branching of the filaments which compose it and in its much 
greater size. The two specimens of this species differ some- 
what from one another in detail. The branching and arrange- 
ment of the filaments is almost precisely similar, but the frond 
of the one § is oblong-oval, cordate, and strongly crenate, while 
the other (Fig. 2 a) is oblong-elliptical with a very slightly 
crenate margin. The midrib is very distinct, and of about 
the same diameter as the upper part of the stalk. The primary 
branches or pinnae are given off oppositely from the midrib 
1 Phyc. Aus., PI. 7 and 32. 2 Linn. Soc. Joum. Bot. vol. xiv. 
3 Harv. PI. 7. 
