1905] PEIRCE & RANDOLPH—IRRITABILITY IN ALGAE 345 
margins. Comparing these two figures with figs. 23, 24, showing 
plantlets of the same age on smooth glass, we see that the plantlets 
have not changed in character from day to day, though they have 
grown; that the rhizoids are still filamentous; and that no holdfasts 
are yet begun. 
ie J 
sowing as fig. 20, 33 days since sowing.—24. Same 
26a “25. Ten days since sowing, a on ground side of cover glass, 6 on unground 
¥ On cover glass.— 26d. ¢ L * 4: at halle J) 
the —— 
well-developed holdfast. 
Fic. 27.—Two Dictyota plantlets on smooth glass. X123- 
into “lowe formed a holdfast on coming into contact with dirt; 6 finding no such obstacle has grown out 
thizoid; a erect, b erect at tip but rhizoid creeping. 
ery al 
In jig. 25 we see the condition of two plantlets of Dictyopteris 
ten days after Sowing, the one on ground glass, the other on smooth. 
hese are not the extremes of two series of plants, but are the average. 
On unground glass, however, under certain conditions, as elaborate 
