460 Peirce. — Studies of Irritability in Plants . 
experimented upon are positively phototropic, growing erect on the clocks 
and toward the light in the shelf-cultures. But, as Anthoceros shows, we 
can distinguish between the directive and the formative influence of light. 
It is also possible to discriminate between the influence of the direction 
from which the light comes, which, in a way, moulds the shape of an 
organism, and the influence of the quantity or intensity of light, which 
affects both the quantity and the kind of growth. Thus cultures too 
feebly illuminated contain thin, long, slender plants (‘ drawn,’ as a horti- 
culturist might say), which do not develop reproductive organs, though 
the plants may be large enough. Light, then, is a necessary stimulus to 
the formation of archegonia and antheridia, and light sufficient for vigorous 
vegetative growth may not be sufficient to stimulate plants to form 
reproductive organs. This, Vochting 1 , Klebs 2 , and their followers have 
shown. But given the necessary quantity or intensity of light 3 , its 
direction will profoundly affect the form of a growing plant, and also 
the positions of the vegetative and other organs which develop upon it. 
This is evidently true of Anthoceros , but is not by any means so evident in 
the cases of Fimbriaria and Gymnogramme. Why? To answer this 
question we must examine the actual working of our clinostats. Unless 
cultures are constantly and uniformly illuminated, the plants do not receive 
at all times equal amounts of light on all sides. Thus, the gradually 
increasing light at dawn finally becomes sufficiently strong to exert 
a definitely stimulating influence upon the plants on which it falls. The 
plants are in a certain position when the light attains this degree of 
intensity. The revolving culture will bring them successively into all other 
possible positions, until finally, at the end of a quarter-hour or of an hour, 
according to the speed of each particular clock, they come once more into 
the position in which they were first stimulated by the light. The time of 
revolution, whether a quarter-hour or even an hour, may or may not be 
less than the time requisite to exert a lasting influence upon the form 
of the young plants. But in the morning the stimulus exerted upon any 
one part is followed by an equal, or even greater, stimulus exerted upon 
all other parts in succession, thus balancing effects on opposite sides. 
This is not the case at sunset. As the light fades, there comes a moment 
when the light still definitely stimulates a growing part, but, the moment 
after, the light does not stimulate the part presented to it by the revolving 
clock. And so it goes the night through, and till dawn again. The part 
last stimulated can react unopposed or unbalanced by other parts. How- 
ever, if the clocks be sufficiently fast, and although the part last stimulated 
1 Vochting, H., Uber den Einfluss des Lichtes auf die Gestaltung und Anlage der Bliithen, 
Jahrb. f. wiss. Bot., xxv, 1893. 
2 Klebs, G., Die Bedingungen der Fortpflanzung bei einigen Algen und Pilzen, Jena, 1896. 
3 For a voluminous discussion of this topic see Mac Dougal’s Influence of Light and Darkness 
on Growth and Development, Mem. N. Y. Bot. Gard. II, New York, 1903. 
