F. Børgesen: Rhodophyceæ of the Danish W. Indies. 



367 



Fig. 359. Eucheuma isiforme (Ag.) J. Ag. 



a, Part of the thallus with tetrasporangia. 



b, transverse section of a branchlet with tetrasporangia. 



(a, about I'/a:!; 6,70:1). 



The specimens found were often more than a foot long. The thallus 

 is of a very cartilaginous consistency; it is terete and, at any rate in 

 the young specimens, oppositely or verticillately ramified. Some of 

 the branches grow out to main branches like the mother branch; 

 most of them remain short as spiny branchlets. 



The summit of the thallus consists of several filaments densely 

 packed together. 



A transverse section shows that the thallus in the middle has a 

 small medullary tissue composed of slender, but thick-walled cells. 

 This is surrounded by a thick parenchymatic tissue whose cells are 

 roundish-polygonal with rather thick walls. These cells are largest 

 innermost growing gradually smaller towards the periphery. It is 

 surrounded by the epidermal layer composed of short radiating fila- 

 ments forming 2 — 3 layers of small, oblong, densely placed cells. 

 At the periphery a rather thick epidermis is present. 



A longitudinal section shows that the cells of the medulla are 

 long, cylindrical. They are twisted between each other and now and 

 then ramified. The cells of the parenchymatic tissue have nearly the 

 same shape as when seen in transverse section. 



Specimens with tetrasporangia and cystocarps are found. Both 

 kinds of organs of fructification occur in the spiny branchlets (Fig. 

 359 a), the tetrasporangia also in the main stems. 



The tetrasporangia occur scattered in the cortical layer (Fig. 

 359 h). This is rather much developed. The cell-threads which form 

 it are longer and consist of more cells than in the vegetative plant. 

 The tetrasporangia are formed as a side-branch from these filaments. 



