454 Peirce . — Studies of Irritability in Plants. 
is true ; but it is the direction of the light that determines the direction 
of the long axis of the germ-tube. By so doing it affects the plane of 
division, indirectly if not directly. In any case, it is extremely probable 
that what appears to be very directly the result of light action, is rather 
the end result of a long series of reactions, only the first one of which was 
due directly to the influence of light. 
In Fig. 3 is shown a group of two Fimbriaria spores, as they were at 
2.15 p.m. on the thirteenth day after sowing. The rhizoid formed by one 
of the little plants had grown out from the side of the plantlet away from 
the light, and was already bending toward the darker part of the culture. 
The same plantlets at 11.15 a.m. the next day are shown in Fig. 4. At 
this time the rhizoid was growing still more markedly away from the light, 
in spite of the fact that for more than half of the time which had inter- 
vened between the two sketches the plants were in darkness, or in extremely 
feeble light. Fig. 5 shows a plantlet, in which the third and fourth division 
walls, as well as the first and second, are nearly at right angles to the 
direction of light. The opaque spore shut off a certain amount of light, 
and hence the direction of the first wall, parallel with the general direction 
of the rays, is really at right angles to the rays actually reaching the 
germ-tube. The figure shows, furthermore, that the rhizoid, as in sessile 
algae, develops on the darker side. 
Although in many cases — in fact, the majority— -the spore puts out the 
germ-tube on the side toward the light, it does not invariably do so. 
Perhaps this is due to the effect of light upon the spore as it matured 
in the tissues of the sporophyte. But the spores which send out a 
germ-tube in other directions than toward the light are relatively so few 
that I cannot believe this to be the case. Perhaps there is such a thing as 
polarity in these spores, but, if so, the manner of collecting and sowing the 
spores should result in a much more general distribution of their poles ; 
their germ-tubes should not originate mainly on any one side, whether 
toward the light or in any other direction. 
Fig. 6 indicates the direction of growth and division of the germ-tubes 
of Gymnogramme , the spores of which I had sown on February 23, and 
which had been almost continuously illuminated by incandescent electric 
lights for five weeks. The slowness of germination may have been due to 
various causes ; but the most evident of the possible influences affecting 
germination was the intensity of the light. For twenty-five days I lighted 
the cultures by 4-candle power bulbs. Finding such slight evidence of 
germination at this time, I changed to 8-candle power frosted bulbs, 
and at the end of three days to 16-candle power clear bulbs. At the 
end of six days very many fern spores were germinating. The tempera- 
ture in the box in which lights and cultures were, was about that of the 
room for the first twenty-five days, and went up to about 32° C. when 
