45 6 
Peirce. — Studies of Irritability in Plants. 
3. Description of the Influence of the Direction of 
Light upon Form. 
Spores of Anthoceros fusiformis were sown on damp soil in the black- 
topped crystallizing dishes previously described, some on clinostats, some 
on the shelf a few inches above them. The sowings were made on 
November 1. The clinostats made a complete revolution once in fifteen 
minutes, and were constantly in motion, by night as well as by day. The 
laboratory was dark between sunset and sunrise. Seven weeks, less one 
day, after sowing I made the drawing reproduced as Fig. 9. This repre- 
sents a young plant taken out of the culture and placed in part profile in 
a drop of water on a slide. The general shape of the plant is that of 
a vase, with a flaring concave top, lobed in outline, borne on a solid base 
tapering to the bottom. From all sides of the base rhizoids grow in various 
directions. The soil on which this plant grew was not sterilized, and the 
plant contained one Nostoc colony of normal size and appearance. If this 
figure be compared with Figs. 10 and 10#, representing a plant growing in 
a dish on the shelf immediately above the clinostat, but otherwise under as 
nearly as possible identical conditions, it becomes evident that there are 
decided differences. Fig. 10 a is an optical section of the plant drawn from 
above in Fig. 10. These drawings show a lobed plant expanded at right 
angles to the light (the arrow indicates the direction from which the light 
comes) and bearing rhizoids on the shaded side only. This is the usual 
form of the ordinary young plant of A . fusiformis whether grown in culture 
or found out of doors. The position of the plant is unusual in one respect ; 
it is nearly vertical to the surface of the soil instead of being more or less 
closely appressed to it. The reason is that the light falls upon the culture 
in a direction nearly parallel to the surface of the soil, whereas out of doors 
the light usually falls at nearly a right angle to the surface. The position 
of the plants out of doors and in cultures is the same in relation to the 
light — the dominant influence — namely at right angles to it. The direction 
of the surface of the soil exerts no influence on the direction of growth 
except as it ordinarily determines the direction from which the light comes. 
Corresponding with the results indicated by the foregoing figures 
of A. fusiformis are the results of sowings of spores of A. Pearsoni , one set 
in dishes revolving on clocks and the other in dishes unmoved on the shelf 
three inches above. Figs. 11 and 11a show plants six weeks, less one day, 
after sowing. The clock was a slow one, making a complete revolution 
once an hour. In spite of this rate of revolution, one quarter as fast as that 
of the clock carrying the plant of A. fusiformis shown in Fig. 9, these 
plants of A. Pearsoni (Fig. 11) are solid, conical, not rolled or flat, flaring 
and lobed at the top, and having rhizoids all around their cylindrical bases. 
The little plant from a culture on the shelf and shown in Fig. 11 a is in very 
