Tilden: pilinia and stigeoclonium. 621 



thalli one or two branches are given off which are so small and 

 thin that one can hardly believe they belong to the same plant. 



Five days later thalli of this stage of advancement had de- 

 veloped numerous upright filaments, among them at intervals 

 a group of threads with as great a diameter as those of the 

 ordinary Stigeoclonium filaments. They branched frequently 

 and these branches terminated in a hair. 



The greater number of the new plates had not as yet pro- 

 duced these raised branches but were still increasing in size. 

 They showed great regularity in the radial arrangement of 

 their prostrate branches. 



A few days later the upright filaments were present in large 

 numbers. The cells near the center of a plate seemed to be 

 first in developing them. 



In fig. 27, a group of the creeping branches is seen, which 

 have not yet become crowded together. One branch just in 

 the act of forming upright filaments is shown from side view. 

 Frequently a basal cell is seen to give rise to two branches 

 lying in the same plane. As observed at (a) these may be of 

 different ages. 



The basal cells are then capable of developing branches in 

 two planes: lateral branches, the cells of which are short and 

 globose or angular, and upright branches, with long cylindrical 

 cells. In either case the branches may be dichotomous or 

 monopodial (PL XXXIV. fig. 27, a and b. ) Therefore in this 

 capacity, the cells of the creeping filaments are exactly homo- 

 logous to the cells of the upright filaments, since the latter 

 also are able to produce dichotomous branching in the plane 

 of the filament or in a plane at right angles to this (approxi- 

 mately) — the plane of the branching. 



Accompanying these upright filaments, there is occasionally 

 to be seen, a long upright bristle (PL XXXIV, fig. 27, c). 



The filaments of the plate when seen in side view are curved 

 in nearly every instance, making the "bow-shaped row of cells" 

 of which Cienkowski (I) speaks. But he takes the view that 

 the convex side of this bow is directed towards the water, 

 while the concave side is turned to the light and that the 

 branches are turned toward the inside, i. e. that they spring 

 from the concave side. A copy of his drawing of this type of 

 thallus is shown (PL XXXIV, fig. 30 J. The germinating fila- 

 ments under investigation show an exactly contrary state of 

 things. The branches invariably take their rise from the con- 

 vex side of the "bow," while the concave side is presented to 

 the surface of the glass (PL XXXIV, fig. 29). 



