Peirce. — Studies of Irritability in Plants. 46 1 
has been stimulated once more than any other part of the plant, there will 
be no visible evidence of this ; the plant will be radial in structure, 
cylindrical or vase-like in form, as my Anthoceros cultures show. 
This result implies two things : first, that the plants do not react very 
promptly to single stimuli ; and, second , that the rate of revolution of the 
clinostat is such that no part of the plant remains exposed so long to the 
light that the stimulus and the reaction of any one part will not be equalled 
on and by all other parts. Both these conditions are realized in my 
cultures of Anthoceros , but not with the other two Archegoniates. The 
thalli of Fimbriaria and the prothalli of Gymnogramme , grown from the 
spore on clocks, are as dorsiventral as in shelf-cultures. This may be 
because the plants owe their dorsiventrality to something else than the 
influence of light ; or, because they are more sensitive, or react more 
promptly to single stimuli than Anthoceros : or> because at sunset the last 
stimulus is sufficient to induce and fix dorsiventrality before the morning 
light can balance this influence. 
As is well known, since Pfeffer’s work on Mar chan tia 1 , dorsiventrality 
once induced cannot be reversed. Nor is it possible to convert one of 
these dorsiventral plants into a radial one 2 , though the change from the 
radial to the usual dorsiventral structure may be promptly made. 
Clinostats revolving faster than any which I have so far employed may 
show which of these possibilities is the fact, or it may be necessary to use 
constant approximately equal illumination from all sides. The former can 
be arranged, and I hope to have such experiments going by autumn. 
Experiments with electric lights were already in progress, and these will 
be resumed presently ; so I hope to be able to report that, with due 
attention to details, experiments on these plants yield results similar to 
those which I have described above for two species of Anthoceros. 
In speaking of the induction of dorsiventrality in the Marchantiaceae, 
Pfeffer 3 says : ‘ Im naheren ist dann immer noch zu entscheiden, ob die 
Dorsiventralitat sich auch bei allseitig gleicher Beleuchtung ausbildet, 
ob also die einseitige Beleuchtung nur raumlich orientirend wirkt, oder ob 
sie eine unerlassliche Bedingung fur die Entstehung der Dorsiventralitat 
ist. Letzteres scheint nach Versuchen von Czapek bei Marchantia der 
Fall zu sein, da sich aus der Brutknospe bei allseitig gleicher Beleuchtung 
(auf dem Klinostaten) im Laufe von 2-3 Monaten kleine schwachliche 
Pflanzchen von radiarem Bau entwickelten. Aus diesen Erfahrungen 
wiirde zugleich folgen, dass in diesem Falle, wie es zu erwarten ist, das 
Zustandekommen der Dorsiventralitat eine Bedingung fur eine kraftige 
Entwickelung ist.’ 
1 Pfeffer, W., Symmetrie und specifische Wachsthumsursachen, Arb. d. bot. Inst. Wurzburg, 
i, 1891. 2 Czapek, loc. cit. 
3 Pfeffer, W., Handbuch der Pflanzenphysiologie, Bd. II, i, p. 182, 1904. 
