CHAPTER VI. THE THALLOPHVTA. 



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forms a flat expansion, consisting of one or more strata of cells, 

 sometimes with a midrib, giving the structure an appearance 

 something like that of a leaf; in other instances tissue-like 

 structures, often possessing considerable complexity, originate 

 from the growing together of adjacent branches ; and in still 

 others true tissues appear to be formed. In the majority of cases 

 growth takes place by the division of a single apical cell, but in 

 the Corallines and their relatives, there are usually several initial 

 cells. This group is also distinguished from the rest by the fact 



that they secrete large quanti- 

 ties of lime in their cells. 



The most distinguishing char- 

 acteristic of the Florideae is 

 their mode of reproduction. In 

 their asexual reproduction they 

 produce non-motile cells, called 

 tetraspores, which, as the name 

 indicates, are usually formed in 

 fours in the mother-cell. This, 

 however, is not always the case ; 

 sometimes there are but one or 

 two, occasionally as many as 

 eight. In the forms which con- 

 sist of branching filaments, the 

 tetraspores are usually formed 

 in the terminal cells of the 

 branches ; in other species they 

 are usually imbedded in clus- 

 ters in the tissues of certain 

 branches, which consequently 

 often acquire a form quite dif- 

 ferent from the rest. Plants 

 which reproduce asexually by 

 tetraspores, do not usually possess the organs of sexual repro- 

 duction, though there are some exceptions to the rule. Fig. 505 

 represents a small portion of the thallus of a species of Plocam- 

 ium highly magnified, showing tetraspores. 



The sexual organs consist of antheridia and carpogonic.. 

 These may both be borne on the same plant, or, as is more com- 



Fig. 505. — Small portion of thallus of 

 a species of Plocamium. one of the Flor- 

 ide.se. showing branches besting tetraspores, 

 t. Magnified about 75 diameters. 



