14 Dansk Botanisk Arkiv, Bd. 3. Nr. 1. 



In this connection, however, I may remark that in nearly all 

 the species mentioned below I have looked in vain for sexual organs ; 

 only in a single species, Acroch. Sargassi, were they found. 



Neither did tetrasporangia occur in the West Indian species 

 collected by me; on the other hand I have previously described 

 a species Acrochætium {Chantransia) bisporum in which sporangia 

 divided by a cross wall into two spores were common and in 

 another species, Acrochætium occidentale, described below some 

 few sporangia divided in the same way were met with. 



In the above-mentioned paper by Bornet the eminent phy- 

 cologist has pointed out that the development and structure of 

 the basal part of the thallus is of great systematic value in a 

 genus consisting of such small plants and, as to the vegetative 

 and reproductive parts of the thallus, often so very similar. In 

 his excellent treatment of the Danish species Rosenvinge also 

 has largely based his differentiation and classification of the 

 species on the method of development of the basal portion. 



In the West Indian material likewise it has been evident 

 that excellent characters may be found in the base of the thallus 

 and I have succeeded in finding not only most of the types 

 mentioned by Rosenvinge but also others coming near to some 

 mentioned by Bornet. It has, however, not always been an 

 easy matter to follow the development of the germinating spore. 

 When several species grow together this is most often impossible ; 

 upon the leaves of an old Sargassum vulgare was found at least 

 six different species and here it was very difficult to clear up 

 the structure of the basal part, and its development from the 

 germinating spore was quite out of the question. Of one species 

 mentioned below I have only succeded in finding a single specimen 

 and have not been able with certainty to state the structure of 

 the base; but as the plant had a very characteristic appearance 

 I think it is entitled to a description. 



When speaking of the basal part I wish also to point out 

 that there are many connecting links from the epiphytic to the 

 endophytic species. In Acrochætium. crassipes the lowermost 

 part is, when growing upon a plant of soft surface, often some- 

 what immersed ; on the other hand when growing upon the spines 

 of Centroceras it seems not immersed at all. 



Upon Avrainvillea nigricans was found an Acrochætium 

 (A. Avrainvilleæ) whose base, composed of short creeping filaments, 

 was fixed to filaments rather deeply sunk in the tissue of the 

 host plant. The tissue of A. nigricans is very loose and open 



