systematic account of the genus Struvea . 273 
The tenaculum consists of a ring of radiating branched 
rhizoids which surround the disc of contact between the apex 
and the wall of the pinna. The mode of formation of this 
seems to be that the tip of the pinnule becomes somewhat 
flattened on the wall of the pinna, and then, from the outer- 
most region of contact, puts out a number of small radiating 
rootlets which creep along the surface of the pinna, and 
branch so as to form a compact rosette-like structure, which 
adheres to the surface of the filament and fixes the pinnule 
in position. 
Careful focussing has led us to believe that these rootlets 
are entirely superficial, never penetrating the cell-wall, but 
adhering to it probably by some process like that by which 
the root-hairs of higher plants adhere to particles of soil. 
These organs appear to be formed only in response to con- 
tact, like the adhesive discs of Ampelopsis , and they are nearly 
universally terminal in position, only a single lateral tenaculum 
was seen among all the species. A transverse wall is nearly 
always formed in the pinnule at a short distance from the 
apex, thus cutting off a sub-globular terminal cell which bears 
the ring of rootlets (Fig. 1 f ). This wall is evidently con- 
nected with the formation of the rootlets, for it seems to be 
formed only when attachment takes place. A possible func- 
tion of this septum is to prevent too great loss of contents in 
case of injury to the delicate rootlets. Fig. 3 e shows a fila- 
ment of S. ramosa bearing two of these organs at its apex ; 
one or two similar cases were seen in 5 . plumosa. 
As occurring in Valonia fastigiata , Harv., Agardh 1 men- 
tions certain structures, which he terms fibulae. These seem 
from his description to have the same function as the organs 
which we have described under the name of tenacula, in our 
opinion a more appropriate word. His figure (Tab. I, Fig. 5) of 
these organs shows that they differ a good deal in appearance 
from those of Struvea , in being lateral and in the different 
character of the rootlets (if one can so call them), though they 
1 loc. cit. p. 94. 
