THALLOPHYTA: ALGAE 



81 



repeatedly forked, with a basal stalk arising from a disk-like holdfast 

 (Fig. 69). It is rather tough and leathery. In some species air bladders, 

 giving buoyancy to the plant, are conspicuous. Growth occurs by means 

 of an apical cell that occupies a notch at the end of each branch. The 



Fig. 69. 

 size. 



Fucus furcatus. Portion of thallus, showing conceptacles, two-thirds natural 



apical cell is complex, having the form of a truncated quadrangular 

 pyramid and cutting off cells in three planes. When the thallus branches, 

 the apical cell divides vertically into two nearly equal parts, each of which 

 becomes the apical cell of a new branch. Internally the thallus is differ- 

 entiated into a firm outer cortex of photosynthetic tissue and a central 

 colorless pith that is rather spongy. The only method of asexual repro- 

 duction is by fragmentation of the thallus. There are no spores of any 

 kind. 



