8 4 



Elementary Biology. 



FIG. 23. PART OF THE TH LLUS 

 OF Fitcus platycarpns. (Thuret.) 



dimensions of the thallus vary extremely ; but it usually 

 attains a length of from one to five or six feet in the group 

 to which F. platycarpus belongs. 



The branching is dichotomous i.e. the growing apex 

 divides into two nearly equal por- 

 tions. All the branches lie in 

 the same plane, and all more or 

 less resemble each other. Each 

 branch consists of a cylindrical 

 core or mid-rib with a lamella 

 on either side, both lamellae 

 lying also in the plane of branch- 

 ing. Every here and there along 

 the lamellae air-sacs or bladders 

 are found, which are, morpho- 

 logically, simply spaces in the 

 cellular tissue. 



If a section of the branch be 

 examined microscopically it will 

 be found to consist of cells which 

 differ in shape and size according 

 to the part of the branch under 

 examination. On the surface the 

 cells are small, spherical, and have 

 no intercellular spaces between 

 them ; the cells of the centre are 

 elongated and branched, and 

 form a loose spongy mass, with 

 necessarily large intercellular 

 spaces. The cell-walls are very 

 thick and mucilaginous in their nature, and swell up readily 

 under the action of fresh-water. Hence the slimy cha- 

 racter of the seaweeds when handled, and especially if ex- 

 amined alter steeping in fresh-water, or preservation in 

 alcohol or spirit. 



Although the colour of Fucus is a dull brown it is not 



A, dichotomous branching ; f, f, 

 fertile branches. 



