FLORIDE.E 385 



cells, which terminates in a dome-shaped apical cell : in the 

 mature parts, the cells of the central series are elongated, as 

 internodes, while at the nodes are inserted brushes of branched 

 multicellular filaments, often terminating in long fine bristles : 

 in the older parts, some of these filaments are closely applied to 

 the internodal cells, thus forming a sort of cortex. 



B. Ptilota plumosa, Ag., a small, dark-purple seaweed, found 

 growing on vertical or overhanging rocks : mount in glycerine, 

 or, if fresh, in sea- water. Observe a well-marked central series of 

 cells, which are individually short, and there is a single apical cell 

 as before : branches are inserted, in one plane, on either side of 

 the central series : in the older portions a cortex surrounds the 

 central cells, and it may be traced as originating partly by out- 

 growth and division of the central series, partly from the bases of 

 the branches. 



C. Chondrus crispus, Stack (Irish Moss or Carragheen), should 

 also be observed as an example of a more massive thallus ; it 

 is very variable in outline, according to the conditions under 

 which it grows : the branching is dichotomous, and the thallus 

 more or less broadly strap-shaped, being attached by a disk-like 

 organ to the substratum. 



Cut transverse sections of the thallus, and mount in pure 

 glycerine : examine under a high power, and note 



1. The superficial cuticle. 



2. The outer cortex, composed of linear series of small cells. 



3. The inner cortex, consisting of larger cells, with swollen 

 cell-walls, which are traversed by protoplasmic threads : these 

 are, however, usually (if not always) intercepted by a highly 

 refractive pit-membrane, the small size of which makes it difficult 

 to trace any sieve-like structure. 



4. A central mass of hyphal tissue. 



Thus there is no central linear series as in the plants previously 

 described : the whole structure approaches rather to that of 

 Fucus (see below). 



D. Corallina officmalis, L., should also be examined as a type of 

 the lime-incrusted Corallines : it is common between the tide- 

 marks on all our coasts. Note the mode of branching of the thallus 

 and its calcareous consistency. Sections should be cut from the 



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