1905] PEIRCE & RANDOLPH—IRRITABILITY IN ALGAE 347 
two hours and a half it grew 0.089™™; and in forty-five hours and a 
quarter it grew’ o.2195™™. These figures converted into rates of 
growth per hour show that the plantlet increased in length at the rate 
of 4.23 per cent. per hour for the first four hours, 4.4 per cent. per 
hour for the next eighteen hours, and 5.82 per cent. per hour for the 
following twenty-three hours. These figures, however, distribute 
the growth over the whole length of the young plant, whereas it is 
mostly at the tip. Since this is the case, the rate of growth in the 
growing part must be much higher. I was not able at the time to 
make measurements to ascertain this rate in Dictyota. Similarly 
a spore of Dictyopteris poly podioides increased in length 360 per cent. 
in three days, or an actual increase in length of o.292™™. 
All of these cases of growth are unusual. The plantlets sent out 
their rhizoids either horizontally or somewhat obliquely upwards, 
at all events without contact with the surface of the glass. Where 
contact occurs, there is no such extraordinary growth unless the 
contact. be with an exceedingly smooth slippery object. There, as 
In water, the rhizoid will continue to elongate. . 
In a spore of Dictyopteris I attempted to measure the length of 
the growing region, the total increase in length, and the distribution 
of this increase among the cells of the rhizoid; but the growth rate in 
this instance has little value, because of the extraordinarily cold 
Weather prevailing at the time. As the laboratory is cold at night, 
the growth rate for twelve hours of the twenty-four must have been 
much below normal. However, I will give the figures. The increase 
in length of the tip cell of the rhizoid was 150 per cent. or 0.078%" 
i twenty-four hours. The increase in length of the cell next behind 
Was 40 per cent. or 0.020™™, The other cells of the rhizoid did not 
lengthen at all. Of the total increase in length 79 per cent. was made 
by the tip cell. This cell had at the outset only 41 per cent. of the 
length of the growing region. The actual zone of growth, however, 
'S only the tip of the cell. Hence these percentages give only a vague 
and quite inadequate idea of how rapid the growth is in the actually 
towing part. 
SUMMARY. 
a The zoospores of Oedogonium, as has long been known, are 
“ensitive to light, the direction of locomotion and the place at which 
they come to rest being determined much more by the direction and 
