97 



A SYSTEMATIC AND STEUCTUEAL ACCOUNT OF 

 THE GENUS AVRAINVILLEA Decne. 



By George Murray, F.L.S., and Leonard A. Boodle, F.L.S. 



(Plates 288 & 289.) 



(Concluded from p. 72.) 



II. — Structural. 



A mature plant of Avrainvillea consists of unicellular filaments 

 repeatedly branched, and more or less interwoven so as to form a 

 stalked or sessile frond above, and a mass of rhizoids below. 



The filaments of the frond are dichotomously branched, and 

 each branch is constricted at or near its base. In A. longicaulis, 

 where the filaments are moniliform, there is nothing to distinguish 

 this basal constriction from the rest of the innumerable constrictions 

 to which the moniliform appearance is due (fig. 2) ; but in A. 

 Mazei, &c., in which the filaments are cylindrical, the basal con- 

 striction forms a very noticeable feature (fig. 6), and recalls 

 Penicillus, Udotea ciliata, &c. 



The divergence of the branches varies a good deal in the 

 different species ; \n A. longicaulis they are very divergent, often 

 about 160° when first formed, but becoming less in the older 

 parts of the frond. In A. im^uana the divergence is generally very 

 small, the two branches being often nearly parallel. In A. longi- 

 caulis the frond-filaments taper gradually towards their free ends, 

 which are often much clearer than lower down. It may be men- 

 tioned here that the rhizoid filaments of all the species have the 

 character of terminating in slender branches. 



The unicellular character of the filaments in most of the species 

 is never or very rarely interfered with, but, in the smaller rhizoids 

 of ^. longicaulis, the wall occasionally becomes so much thickened 

 that the constrictions become septa. The opposite walls at the 

 point of constriction approach one another until a small canal 

 remains, and this becomes obliterated by the further growth of the 

 walls (fig. 4). In ^. covwsa the cavity of the frond-filaments 

 becomes intercepted at the basal constrictions by the formation of 

 stoppers, which, in appearance and mode of formation, exactly 

 resemble those of other SiphonecB, e.g., Codium and Bryopsis. These 

 stoppers are represented in Sonder's figure, Alg. Trop. Austr. tab. vi. 

 fig. 7. In this species the constrictions are only slight, and are situated 

 a short distance above the bifurcation ; but their position is made 

 conspicuous by the stoppers (fig. 12), which may be seen in many 

 stages of formation. In A. ccespitosa we observed a few incomplete 

 stoppers. 



The cell- wall is uniformly thin, except in the rhizoids of some 

 specimens of A. longicaulis, where it may be very much thickened, 

 as mentioned above. Especially near the apices of filaments, the 

 cell-wall can frequently be distinguished only where the proto- 

 plasmic sac has shrunk away from it. 



Journal of Botany. — Vol. 27. [April, 1889.] h 



