Ewart, — The Effects of Tropical Insolation, 447 
induce in the chloroplastid a condition characterized by a tem- 
porary inability to assimilate. Thus when chloroplastids are 
subjected for prolonged periods to illumination by sunlight 
of natural intensity, though at first they continue to assimilate 
normally, their power of performing C0 2 -assimilation is finally 
more or less markedly inhibited. Pringsheim 1 has shown that 
chloroplastids when exposed to intense and concentrated sun- 
light are killed and bleached only when oxygen is present. 
When subjected to such exposure in an atmosphere formed 
by an assimilatory mixture of hydrogen and CCb the chloro- 
plastids remain green. It follows, therefore, that when exposed 
to concentrated sunlight, no assimilation of C0 2 or evolution 
of oxygen takes place. Hence there appears to be a maximal 
intensity of illumination at which the synthetic assimilatory 
processes are inhibited and cease, being finally replaced if 
a supply of oxygen be present by katabolic oxydatory 
changes. This appears to be in analogy with the general 
law, exemplified more especially in the effects of temperature 
upon the combustion of gases, that combinations between 
substances taking place at a relatively low temperature tend 
to be less and less complete as the temperature increases, and 
finally may be replaced by dissociatory changes taking place 
in the reverse direction. 
Protective Movements by the Leaves or Leaflets. 
Of the various modes by means of which tropical plants 
protect themselves against the injurious effects of over- 
exposure to sunlight, perhaps the most interesting and com- 
monest is by means of active or passive movements of the 
leaves. It may be stated as a general rule, to which 
apparently there are no exceptions, that no tropical plant 
places or allows its leaves to be in such a position that the 
upper surfaces are at right angles to the sun’s incident rays 
when at the zenith. In most exposed tropical plants, in which 
the leaves or leaf-stalks are not stiff and rigid, the leaves at 
1 Pringsh. Jahrb., Bd. xii, 1882, pp. 326, 344. 
