459 
Peirce . — Studies of Irritability in Plants. 
but I will here record the results yielded by an experiment made during 
a two months’ stay at the Zoological Station in Naples 1 . Spores of Dictyo - 
pteris polypodioides were allowed to escape in sea-water in crystallizing 
dishes, the covers of which had sheets of black paper glued upon them. 
Two of these dishes were put on clock works which revolved them once an 
hour in a horizontal plane. The others were placed on the window shelf be- 
side them. These spores develop at first protonema-like filaments, as Reinke 
showed 2 , on which, during the first two weeks, erect and conical buds appear. 
At the end of two weeks or so a flat plate begins to grow from near the top 
of these little balloon-like buds. This is what ordinarily happens in shelf 
cultures, and is presumably what happens in the sea. Fig. 17 shows the 
usual young plant. On the other hand in clock cultures the course of 
events is decidedly different, as shown by Fig. 17 <2. This figure was drawn 
from a plant at the centre of a dish on a clock, the spores being sown 
twenty-three days before the drawing was made. It is evident that no 
part of the plants had become flat, although branching had occurred. 
Fig. 17 indicates that, under ordinary illumination from one side only, the 
young plant soon sends out a branch which, growing upward, is thin, flat, 
and leaf-like. 
Certain species of Fucus , common between the tide-marks on rocky 
portions of this coast, are flat and expanded. Spores of these species 
germinate in dishes of sea-water in the laboratory, as I know by experience. 
As soon as possible I shall try to cultivate these on clocks and on the shelf, 
and I hope to follow the experiment which promised such interesting results 
but which was cut short by the brevity of my stay in Naples. I record the 
experiment here merely because the preliminary result is so consistent with 
the results of my experiments on land-plants, not because I think it 
conclusive or more than suggestive and promising. 
4. Discussion of the Influence of Light upon Form. 
The results previously described show that certain dorsiventral 
Archegoniates and one marine alga are more or less influenced as to their 
form by the direction from which the light falls upon them as they grow. 
This influence is most evident in the case of Anthoceros , which, whether 
growing on a slow clock or on one which revolves four times as fast, 
develops with radial instead of dorsiventral structure. All of the plants 
1 The main results of my work are reported in the paper on Irritability in Algae previously 
referred to. In that paper I took occasion to express my grateful appreciation of the opportunity of 
working at one of the tables supported at the Zoological Station by the Carnegie Institution of 
Washington. I am glad to have this additional opportunity to thank the officers of both institutions 
for their help and courtesy. 
2 Reinke, F., Entwickelungsgeschichtliche Untersuchungen liber die Dictyotaceen des Golfes von 
Neapel, Nova Acta Leopold. Acad., 1, 1878. 
