566 DR. A. J. EWART ON ASSIMILATORY INHIBITION, 
presence of an excess of starch, and the fact that the leaves are 
young and not adult, act as factors delaying the commencement 
of assimilation on exposure to light and rendering it uncertain 
whether under more favourable conditions the etiolated leaves 
might not be capable of weak assimilation. To eliminate these 
errors, plants with amyliferous etiolated shoots were kept in well 
oxygenated water in darkness at 20° C. for 10 days or at 30° C. 
for 7 days, and then examined and exposed to light. 
Elodea canadensis.—Shoots elongated, leaves larger; starch 
grains much reduced in quantity and size, in parts almost none; 
chloroplastids colourless to pale yellow, but no power of assimila- 
tion. After 8 hours’ exposure to light the colour is deeper, but 
still no evolution of O ; at end of 2nd day, chloroplastids yellowish 
to pale yellowish green: from latter a weak evolution of O, from 
former none. At end of 3rd day leaves green to greenish yellow 
and weak to moderately active evolution of O. 
If closed-ringed cell preparations of etiolated leaves of Elodea 
are exposed to light in no case do they turn green, nor if they 
already have been exposed long enough to possess a slight greenish 
tinge does the green coloration deepen in the closed cell. Such 
preparations may remain living for days, and in some cases à 
few of the cells may, if the ringing be thin or if the leaves are 
slightly greenish, show protoplasmic streaming. Correns * has 
shown that for etiolated plants to turn greena given pressure of 
oxy gen, varying from 30 to 60 mm. hydrogen pressure, is required; 
whilst Clark t found that streaming in cells commences at an 
oxygen pressure of from 1:2 mm. to 2:8 mm. hydrogen pressure. 
In the etiolated leaves of Elodea also, under the above conditions, 
a partial pressure of oxygen which is sufficient to permit of slow 
rotation is insufficient to allow the chlorophyll grains to turn 
green, especially as the accumulation of the respiratory CO, 
also exercises an inhibitory influence upon the turning green of 
etiolated leaves f. 
If etiolated leaves are placed in a plasmolytic (20°/,) solution 
of cane-sugar and exposed to light at 20? C. to 25? C., after 3 days 
many leaves are distinctly greenish or yellowish green. 
* C. Correns, in Flora, 1892, p. 141. 
+ J. Clark, in Ber. d. D. Bot. Gesell. vi. 1888, p. 273. 
1 J. Boehm, * Ueber den Einfluss der Kohlensaure auf das Ergrünen und 
Wachsthum der Pflanzen " (Sitzungsber. d. k. Akad. d. Wiss. Wien, Bd. lxviii. 
July 1873), found that cress seedlings in 20 °/, CO, do not turn green, but 
that in 50 °/, in the presence of O, exposed to light, grasses still turn slightly 
green. 
