126 PLANT LIFE. 



mentous ; that is, consisting of numerous long slender 

 branches, each of which grows by the repeated division 

 of an apical cell, of a cylindrical form, with the distal or 

 free end rounded. When the apical cell has reached a 

 given size, a daughter-cell, in the form of a very thin disc, is 

 cut off from its proximal or attached end by a transverse 

 septum. Contemporaneous with the formation of this 

 discoid daughter-cell, a varying number of pericentral 

 cells, depending on the species, are cut off at equal dis- 

 tances round its margin by the development of septa, 

 which curve inwards for some distance. At this stage a 

 transverse section of the discoid cell presents the appear- 

 ance of a cog-wheel, only the notches in the wheel are still 

 filled in the section by the pericentral cells that are separated 

 from the central cell by septa. When the above-described 

 condition has been reached, the central cell, with its sur- 

 rounding cells, increases rapidly in length, until the normal 

 size is attained. By the constant repetition of this process, 

 the branch increases in length, and consequently consists of 

 a series of bundles of superposed axial and peripheral cells 

 of equal length. All the peripheral or pericentral cells are 

 united to the parent axial cell by strands of protoplasm, and 

 also laterally with each other ; the axial row of cells also 

 shows very clearly protoplasmic continuity. 



In many species constructed on the- above plan, the 

 central axis consists of more than one row of cells ; the 

 pericentral cells also by further division may form more 

 than one row. 



The genus Batrachospermiim, a fresh-water type of the 

 Floridese, common in our streams, illustrates in a primitive 

 manner a type of thallus development not uncommon in the 

 Floridese. In the species of Batrachospermum the branches 



