462 Peirce. — Studies of Irritability in Plants. 
As I have shown above, equal illumination is not perfectly attained 
by the use of the clinostat alone. Illumination, to be equalized by a 
cliriostat, must also be constant. This, sunlight is not. Artificial lights 
may be used night and day, but their composition is not altogether like 
that of sunlight. The use of artificial light, which I have begun, involves 
the question of which rays are the more potent in influencing the forms of 
plants. To this question I shall give attention. But, so far as my 
experiments have gone, it seems that the usual dorsiventrality of 
Anthoceros may be replaced by a remarkably regular radial structure 
whenever the light is made to fall, even in approximately equal strength, 
on all sides successively. If Anthoceros plants, started under equal 
illumination from the spore, can be made to grow at all in darkness, 
their behaviour will be very significant. If they become dorsiventral 
under these conditions, it would seem that the tendency to become dorsi- 
ventral is inherited, and was held in check only by abnormal illumination. 
If they continue to be radial and yet grow, it would seem that dorsi- 
ventrality is not inherited, but is induced by certain conditions, and it 
would also seem that a certain factor in the environment, the light and 
its direction, constitute this condition. To determine whether light from 
one side only is a condition merely, or is a necessary stimulus to 
dorsiventrality, is more difficult. This question can be answered by 
employing suitable artificial lights, perhaps incandescent electric bulbs 
of equal candle power, and so disposing them that cultures are equally 
lighted on opposite sides by two or by four lights. If with equal 
illumination on two opposite sides, the young plants are dorsiventral, 
Pfeffer’s suggestion 1 — that ‘ lighting from one side is a condition necessary 
for the development of dorsiventrality ’ — would seem superfluous, and 
that the light acts rather as a definite stimulus, the direction from which 
the light comes being also significant. These experiments had also been 
begun, but had yielded no definite results before the earthquake stopped 
my work. 
The significance of such an investigation as this, the early results of 
which I have reported in the foregoing pages, consists not merely in the 
matters of fact thus revealed, but also in the light these facts throw on the 
problem of heredity. Every biologist is driven sooner or later to a con- 
templation of one aspect or another of this problem. The continuity of 
substance from parent to offspring constitutes, according to most persons, 
so very much the principal basis of heredity that it may almost be called the 
sole one in their estimation. Such biologists attempt to express in morpho- 
logical or in other definite terms the means of transmitting characters from 
one generation to the next. Thus by chromosomes or germ-plasm or some 
other name the part or parts are designated to which especial importance 
1 Loc. cit. 
