HOW TO KNOW THE SEAWEEDS 





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Fig. 7. Some examples of different kinds of branching in the marine algae. A. 

 Unbranched (simple); B. Dichotomous C. Pinnate alternate; D. Pinnate opposite; E. Ver- 

 ticiHate; F. Multifarious; G. Pectinate, or secund; H. Monopodia!; I. Sympodial. 



Other filamentous thalli will be found to be multiseriate, or made 

 up of several rows, tiers or layers of cells. Sometimes it will be pos- 

 sible to interpret the structure satisfactorily by focusing through the 

 more or less transparent external cells to observe the inner ones, but 

 if there are more than two or three layers of cells involved it usually 

 will require a cross section or longitudinal section to determine the 

 cell forms and relationships. Such sections will often result in the 

 discovery that the filament which appeared to be soHd is actually a 

 hollow tube, or that the central region contains a distinctive kind of 

 cell structure which was invisible from the outside. 



Coarser thalH will be found to be made up in a variety of differ- 

 ent ways. Some quite large plants such as Codium fragile may con- 

 sist entirely of branched, unseptate, coenocytic filaments, while sUp- 

 pery or gelatinous plants such as ^Nemalion often are similarly com- 

 posed of branched filaments which are, however, regularly septate. 

 Others, such as Gracilaiia, show a firm, soUd, compact structure of 

 cells of increasing size from the outside inward. Many of the larger 

 thalh, either cyUndrical or flattened, show a differentiation of tissues 

 whereby a parenchymatous medulla is enclosed by a filamentous cor- 

 tex, or vice versa. In some of the giant kelps, such as Nereocystis, such 

 specialized structures as sieve tubes comparable to those of higher 

 plants occur. 



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