344 BOTANICAL GAZETTE [NOVEMBER 
cover glasses ground on one side only, some with the ground surface 
upward, others with the ground surface downward, served as the 
bottom on which the spores settled. 
In these experiments I used the same species previously enumer- 
ated, but those which form a single rhizoid at first give more imme- 
diately recognizable results than those which, like Cystoseira, form 
several rhizoids. For this reason Dictyota, Dictyopteris, and Lauren- 
tia are preferable. I shall describe first Dictyopteris, as it was the 
marine alga with which I first obtained satisfactory results. The 
results obtained with Laurentia are so similar that I shall not speak 
of them further. 
In fig. 19 we have sketches of Dictyopteris plantlets two and five 
sixths days after sowing on ground glass. Fig. 20 shows plantlets 
sowed at the same time on smooth glass. In the first set the rhizoids 
are beginning to enlarge, and one to branch, at the tips. In the 
latter there is no such appearance. The idea that this difference in 
appearance might be due to a difference in age suggests itself. But 
the difference in age could be only slight, for the fertile branches were 
removed at the same time from both dishes, and both dishes ares 
emptied and filled with fresh sea water at the same time. Emptying 
the dishes generally removes the youngest spores, those still unattached 
or only slightly so, leaving those which are equally firmly attached " 
I am nevertheless inclined to think that not merely is growth in the 
part of the rhizoid in contact with a rough surface stimulated by that 
contact, but also the growth and development of the other parts of 
the plantlet also. There seems to be a transfer from cell to cell of 
the stimulus produced by contact, a stimulus the greater the rougher 
the surface. If this be true, there is every reason for the younge 
appearance of the plantlets on the smooth glass, for their growth Lg 
less stimulated than that of the other plantlets. This 182°” 
with the observations previously referred to (10) which Loeb 
upon animals. 
Fig. 21 shows the same group of plantlets nearly twenty es 
later. Here holdfasts are evidently begun. Twenty ours poe 
this we have the condition shown in fig. 22, where the rhizois : 
almost completely lost their filamentous appearance and oa > 
holdfasts, circular in general outline, but with elaborately | 
