THE ALGAE 325 



Attention must also be drawn to the fact that within the various 

 classes and phyla a number of definite morphological tendencies 

 can be recognized, and these are repeated within the groups. In 

 view of the plasticity of primitive organisms it is not surprising that 

 the various potential Hues of development have been exploited 

 more than once. Examples of this parallelism within the simpler 

 algae are provided in Table 6 (p. 325) and for the more advanced 

 algae in Table 7 (p. 326). 



A word may conveniently be said about the time when the differ- 

 ent algal groups made their first appearance. Many authors would 

 consider that the Myxophyceae and Chlorophyceae are the most 

 primitive and therefore appeared first. If, however, the Rhodo- 

 phyceae and Phaeophyceae have a flagellate origin then all four 

 groups may be of the same antiquity. Whatever the sequence of 

 events, it is quite clear from the structure of the earliest fossil 

 algae (see pp. 296, 298) that considerable evolution had taken place 

 long before even their time. 



One further point remains to be added. In the present chapter it 

 has been suggested that the similarity in life cycles and a study of 

 phylogeny leads one to the hypothesis that the Chlorophyceae, 

 Phaeophyceae and Rhodophyceae may be interrelated (hence their 



inclusion in the Euphycophyta). At the same time it has been in- 

 dicated that other workers beheve that all three groups have had 

 independent origins from different sources. The final decision be- 

 tween these two viewpoints must, with our present knowledge, be 

 largely a matter of opinion. 



Table 7 

 Parallelism in evolution among the advanced type of algae 



Type of Construction 

 (i) Heterotrichous thallus 



(2) Discoid 



(3) Crusts or cushions 



(4) Elaborated erect type 



(5) Uniaxial pseudo- 



parenchymatous 



(6) Multiaxial ditto 



(7) Foliose parenchymatous 



Chlorophyceae Phaeophyceae Rhodophyceae 



* Stigeoclonium 

 *Protoderma 



Pseudoprings- 

 heimia 

 *Draparnaldia 



Dasycladus 

 *Chara 

 *Codium 

 *Ulva 



*Ectocarpus 

 Ascocyclus 

 Ralfsia 



Sphacelaria 

 *Mesogloia 



Eiidesme 



Petalonia 



Asperococais 



* Batrachospermum 

 Erythrocladia 

 Hildenbrandtia 



Plumaria 

 *Polysiphoma 



Nemalion 

 *Porphyra 

 Halosaccion 



(8) Tubular parenchymatous *Enteromorpha 



* It is recommended that elementary students only remember these examples. 



