HOW TO KNOW THE SEAWEEDS 



may occur mostly by fragmentation of the filament, but more com- 

 monly some kind of spore is produced which may germinate to pro- 

 duce a new plant. Two common kinds are the motile, flagella-bearing 

 zoospore, and the non-motile aplanospore. These may be produced 

 simply by a differentiation of the contents of a vegetative cell, or, if 

 the structure producing the spores is in any way specialized and dif- 

 ferent from the ordinary vegetative cells, it is called a sporangium. 



Sexual reproduction in the Green Algae involves a union of gametes 

 which may be motile or non-motile. In the great majority of marine 

 green algae the gametes are flagellated. When fusion occurs between 

 those which are of equal size the species is called isogamous, while 

 union of flagellated gametes of unequal size is characteristic of aniso- 

 gamous species. 



A type of life history which may be considered fairly representa- 

 tive of the many larger marine green algae is exemplified by Ulva, the 

 sea lettuce. In Ulva, an alternation occurs between an asexual gen- 

 eration (sporophyte) producing quadri-flagellate zoospores, and a sexual 

 generation (gametophyte) producing bi-flagellate gametes. The alter- 

 nation is called isomorphic because the sporophyte plant (diploid gen- 

 eration) is essentially identical in external appearance with the game- 

 tophyte plant (haploid generation). The life cycle is diagrammed in 

 Fig. 9. 



UnUke Ulva, some genera of marine green algae bear special struc- 

 tures (sporangia and gametangia) for the production of their reproduc- 

 tive cells, but students will find that most genera of green algae can 

 be recognized so readily from their vegetative form that reproductive 

 organs usually need not be present for identification to that point. 

 Specific identifications, on the other hand, may often require careful 

 study of the reproductive organs and of the Hfe history, sometimes 

 even to the point of culturing the plants to obtain Uving spores and 

 gametes for examination. 



THE BROWN ALGAE 



As a broad generalization it may be said that all the brown algae, 

 with the exception of the Fucales, have an alternation of sporophyte 

 and gametophyte generations, but among the greatly diversified forms 

 of this large marine assemblage a number of variations and compli- 

 cations in the Hfe histories are superimposed upon this fact. On the 

 other hand, the beginning student will be relieved to know that the 

 majority of the large brown, seaweeds have only one general kind 

 of Hfe cycle with which he needs first to become familiar. In this great 

 majority of instances the large, macroscopic plant which is collectable 

 in the field is the sporophyte generation which alternates with a micro- 



21 



