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



Dictyerpa Collins. 



40. Dictyerpa Jamaicensis Collins. 



Collins, F. S., The algæ of Jamaica (Proceed. Americ. Acad. of Arts 

 and Sciences, vol. 37, 1901, p. 251). 



Some small specimens have been found which I think refer- 

 able to this plant. They were collected in a rather exposed place 

 upon the small reef near the entrance to the harbour of St. Tho- 

 mas. They form small, low tufts, most probably growing in nar- 

 row crevices in the rocks over which the waves constantly dashed. 

 They are fixed to the substratum by means of numerous rhi- 

 zoids breaking out in groups everywhere on the thallus. 



The thallus consists of thin slender filaments about 300 to 

 600 |i thick or more, which, in transverse section, are roundish or 

 oval. The ramification is very irregular, being di, tri- to polychoto- 

 mous. The internodes are of variable length; they are thinnest 

 at their base and increase gradually upwards. The young group 

 of rhizoids are covered by the cuticula forming an indusium which 

 bursts later on. The rhizoids are about 27 |a thick, being divided 

 into cells more than four times longer. They are irregularly bent 

 and nearly destitute of contents. 



The thallus increases by means of a large nearly hemisphe- 

 rical apical cell from which segments are cut ofT in all directions. 



From a transverse section is seen that the thallus consists 

 of a cortical layer of small, nearly quadrangular cells with con- 

 siderable contents and a medullary layer of larger colourless 

 cells being irregularly polygonal or often nearly rectangular; 

 the walls of these cells are more or less undulated. A longitudinal 

 section shows these cells to be a1)out twice as long as broad. 



Regeneration seems to take place very easily, I have several 

 times seen a group of young branches grow out from the thallus 

 when it has been broken. 



As in the case of the plant from Jamaica this, too, was quite 

 sterile. 



Regarding this plant Svedelius in Engler u. Prantl, 

 Nat. Pflanzenfam., Nachtr. zu I.Theil, Abt. 2, p. 188 writes: Die 

 Gattung Dictyerpa ist höclist wahrscheinhch nichts anderes als 

 eine freiliegende, trotzdem aber weiterlebende Form einer nor- 

 malerweise auf Steinen wachsenden Dictyota, die durch die 



I 

 I 



