Peirce. — Studies of Irritability in Plants. 457 
sharp contrast to these, being flat and expanded at right angles to the 
incident rays of light, and having rhizoids on the shaded side only. 
Similar cultures of Fimbriaria Calif ornica are shown in Figs. 12 
and 12 a. Fig. 12 shows plants growing on a quick clock, revolving com- 
pletely four times an hour, and drawn five weeks and one day after sowing. 
These plants are solid, erect, with conical more or less lobed and concave 
tops borne on cylindrical bases, from all sides of which rhizoids spring. 
Fig. 12 a shows a shelf plant of the same age, with the ordinary form, 
expanding at right angles to the light, and with rhizoids only on the shaded 
side. Fig. 13 shows a plant midway between Figs. 12 and 12 a in form. 
This plant grew on a slow clock, making only one revolution per hour. 
The plant was drawn five weeks and a half after sowing. The plant is 
going over from the vasiform to the usual flattened thallus. This change 
is completed a little later. Indeed on all my clocks, fast or slow, Fim- 
briaria plants lose their early cylindrical form sooner or later. Whether 
grown on clocks or shelves these plants are all cylindrical at first. The 
only difference made by my clocks is to defer the period of flattening. 
The clocks defer this for a longer or shorter time according to their speed, 
the faster the clock the more perfectly round the plant and the longer 
it will remain so. I have as yet no means of revolving cultures more 
rapidly than four times an hour, but I hope to have on resuming these 
experiments next September. 
Cultures of Gymnogramme triangidaris five weeks and a day after 
sowing are shown in Figs. 14 and 14 a. Although spores were sown in 
a culture on a quick clock, the young prothalli of this fern did not remain 
cylindrical in form beyond the very early filamentous stage. I shall try 
this again, however, when I have the means of more rapidly revolving 
cultures, and also of exposing stationary cultures to equal illumination 
on all sides. Figs. 14 and 14 a show plainly, however, that the light deter- 
mines which side of the thallus shall develop rhizoids, and where the 
illumination is approximately equal on all sides rhizoids develop on the 
various sides of different prothalli. Fig. 15 shows a prothallus seven weeks 
after sowing on a slow clock. The prothallus is thin and expanded, though 
its edges are somewhat rolled, and it bears approximately equal numbers of 
rhizoids on its two faces. Furthermore, antheridia and archegonia develop 
equally on the two sides. It is perfectly possible to suppress the develop- 
ment of the reproductive organs of this fern by insufficient light, but when 
the illumination is approximately equal and is sufficient on all sides, the 
antheridia and archegonia develop in spite of there being no shaded side. 
The advantage of their developing on the shaded, usually the lower, side is 
evident, but the reason for their doing so is far from clear when it is possible, 
as just stated, to suppress their formation altogether by feeble illumination. 
For one reason or another, the final and most compelling being the earth- 
