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



345 



At the base the stem produces irregular bundles of rhizoids, 

 which gradually separate, fixing themselves to stones, shells, coralline 

 algæ etc. 



The single leaf of the thallus is linear-lanceolate of shape with 

 an obtuse, generally emarginate apex. It consists of only a single layer 

 of cells, with the exception of the midrib in which three layers are 

 found (Fig. 342 C), namely a smaller, nearly quadratic central cell 

 and two larger, peripheric cells. 

 From this midrib proliferous 

 young leaves are repeatedly 

 issued (Fig. 340), the ramifi- 

 cation of the thallus being 

 formed in this way. 



In the older part of the 

 thallus the midrib and nearest 

 surrounding cells of the leaf 

 become strengthened by rhizo- 

 ids growing out from the basal 

 cells of the proliferous leaves. 

 Gradually in approaching the base these rhi- 

 zoids form a parenchymatous tissue which becomes 

 thicker and thicker until it, as mentioned above, 

 becomes in strong plants nearly semiglobular in 

 the basal stem (Fig. 341 a). 



The thallus increases by means of an apical 



cell from' which semicircular segments are cut off Fig- 340 DeZesserm 



^ tenuifoha Harv. 



(Fig. 341 b). The segments are divided m a cen- part of a female 



tral and two marginal cells. It is a well known plant with crysto- 



T ^ c^ carps, 



fact, mentioned already by Nägeli and Schwende- (About 4:1). 



NER*), that the thallus of the Delesseriaceæ is built up 

 by means of filaments congenitally connate. From the central axis 

 opposite, biseriate filaments aie issuing. These filaments are unil- 

 aterally ramified, the branchlets issuing from their underside. 

 In our plant, cp. Fig. 341 b, the first side-branch is usually issuing 

 from the large cells on both side of the midrib and, sometimes, from 

 one of the following cells in the filaments still one more side-branch 

 is given off in other filaments the branching first takes place from the 



Das Mikroskop, 2te Aufl. 1877, p. 561. 



