422 BOTANY PART n 



The thallus of the Red Algae exhibits a great variety of forms. 

 The simplest forms are represented by branched filaments consist- 

 ing of single rows of cells (e.g. Callithamnion). In other cases the 

 branched filamentous thallus appears multicellular in cross-section. 

 In many other forms the thallus is flattened and ribbon -like (e.g. 

 Chondrus crispus, Fig. 363 ; Gigartina mamillosa, Fig. 364) ; while 

 in other species it consists of expanded cell surfaces attached to a 

 substratum. All the Florideae are- attached at the base by means 

 of rhizoidal filaments or discoid holdfasts. One of the more com- 



Fio. 363. Chondrus crispus. (i nat. size.) 



plicated forms is Delesseria (Hijdrolapathuni) sanguined (Fig. 88), which 

 occurs on the coasts of the Atlantic. The leaf-like thallus which 

 springs from an attaching disc is provided with mid-ribs and lateral 

 ribs. In the autumn the wing-like expansions of the thallus are 

 lost, but the main ribs persist and give rise to new leaf-like branches 

 in the succeeding spring. The thalli of the Corallinaceae, which 

 have the form of branch -systems or of flattened or tuberculate 

 incrustations, are especially characterised by their coral-like appearance, 

 owing to the large amount of calcium carbonate deposited in their cell 

 walls. The calcareous Florideae are chiefly found on coasts exposed 

 to a strong surf, especially in the tropics. 



