WHAT TO LOOK FOR 



N beginning a study of the seaweeds the student usually 

 expresses surprise that the members of a single family 

 or genus appear to be entirely different from each other, 

 while, on the other hand, members of completely unre- 

 lated groups may look aUke. This is so because his su- 

 perficial examination of the plants has not permitted him to see the 

 features by which the plants may be related or distinguished. It must 

 be emphasized that for the beginner, who has as yet no acquaint- 

 anceship with the various forms, microscopic examination of the 

 vegetative structure and also of the reproductive organs of the plants 

 is essential to an understanding of them and to his success in the use 

 of the key which follows. 



VEGETATIVE STRUCTURES 



Among these macroscopic marine algae with which we are deal- 

 ing, the seaweeds, the protoplasm is always surrounded by a cell wall 

 which may assume a multitude of different forms and which may 

 range from thin to thick, and from rigid to gelatinous. In a great many 

 of the smaller algae, or even larger forms whose thalU are finely dis- 

 sected, much of the cellular structure may be observed simply by 

 making a whole mount of a piece of the thallus on a slide. The larger 

 and coarser forms, however, have such a dense structure that it is 

 impossible to make out details of cellular structure unless thin, trans- 

 parent sections are cut to show the organization of the cells in the 

 plane desired. 



Many of the delicate forms will show under the microscope that 

 they are made up of a single row of cells constituting a uniseriate fila- 

 ment. Some such filaments are always unbranched, while others are 

 branched in various ways. The diagrams in Fig. 7 show several dif- 

 ferent manners of branching which may be encountered. 



Among uniseriate filaments, some will be encountered which have 

 short cells, about as long as wide, while others will have elongated 

 cells (See Figs. 96 and 97). Still others will exhibit only occasional 

 cross walls, often only at the points of branching. These will be coe- 

 nocytic forms of marine green algae in which the cells are multinu- 

 cleate. Among these coenocytic green algae are a number of forms in 

 which cross walls are rare or entirely lacking in the filaments, so that 

 the entire plant consists of a variously ramified hollow tube. The fam- 

 ous Valonia ventricosa (See Fig. 13) consists essentially of a single 

 large, multinucleate cell which may become as large as a hen's egg. 



16 



