Peirce. — Studies of Irritability in Plants. 455 
I put in the 16-candle power lights. This may have accelerated germination ; 
but the experiment, except as indicating that the direction of growth and of 
cell-divisions in the germ -tubes of this Fern is influenced by light, even of 
composition so different from sunlight, was of little value. A repetition 
of the experiment was interrupted by the earthquake of April 18. 
It is conceivable that in all of these cases the side from which the 
spore puts out the germ-tube, the direction of growth of the germ-tube, 
and the side of it on which the first rhizoid appears, may be the sides or the 
direction most free from the mechanical pressure or resistance of particles 
of soil or of raised spots in the filter-paper on which the spores were 
sown. To avoid this objection, I sowed spores of Gymnogr amine on the 
surface of a filtered Knop’s solution in two covered crystallizing dishes. 
Black paper was glued to the covers, so that light reached the spores only 
through the sides of the dishes. One dish I placed on aclock-clinostat, making 
a complete revolution every quarter-hour ; the other dish was on the shelf 
three inches above. Fig. 7 shows two young Ferns, sixteen days after the 
spores were sown in the dish on the shelf. The light fell in the direction 
indicated by the arrows. The germ-tubes are growing toward the window, 
the first division walls are at right angles to the incident rays, and the 
rhizoids appear on the dark side of the young plants. Fig. 8 is a diagram 
in which is indicated by arrows the direction of the germ-tubes of the 
spores germinating on the Knop’s solution in the dish upon the clinostat. 
The age is the same, all the conditions were the same for the two cultures 
except that in this culture the light entered through all sides successively 
as the dish was revolved. The direction of growth of the rhizoids was as 
various as that of the germ-tubes, generally, however, opposite to that 
of the germ-tube. These two cultures show plainly, therefore, that it is 
the direction of illumination, and not any mechanical resistance, which 
determines the direction of growth of the germ-tube and of the first 
divisions of its cells, and that ordinarily the rhizoids spring from the shaded 
side of a plantlet, and grow away from the light. 
Thus we see that, in respect to their relations to light as an influence 
directing growth, the spores of these archegoniates behave similarly to 
the spores of the algae previously described. The advantage is obvious : 
the germ-tube grows where light will enable the young plant to make food 
for itself ; the rhizoid grows where it can form an attachment for the young 
plant. At this stage at least, water and food materials are absorbed 
through all parts of the plant-body ; the main function of the rhizoid is to 
fasten the plant to the soil, and, by giving it suitable mechanical support, 
enable it to continue the growth of the green parts in the most 
advantageous direction. This is probably the main function of the 
rhizoid throughout the lifetime ol these small plants ; but it may not 
be the sole function. 
