FLORIDELE. 41 



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 officinalis, 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 

 tissues, either after dissolving the lime with acetic acid, or, 

 better, by grinding as in the preparation of geological sections : 

 in the latter case observe 



1. The successive pear-shaped joints of lime-incrusted tissue : 

 these can be recognised externally with the naked eye. 



2. The intervening transparent zones of tissue with pliant 

 mucilaginous walls. 



