in Plant Cytology. 195 
again may ultimately be surpassed by physico-chemical 
results. Meanwhile it is needless for us to attempt reaching 
the last hastily, except by the bridge which function has built 
for us. No function in animals is of more prime importance 
than that concerned with the taking up and propagation of 
external stimuli. Through it has been called forth the com- 
plicated nervous system, which brings each animal into proper 
relation to its environment, and enables it to react on this. 
The investigations into plants along similar lines are still com- 
paratively few, but largely owing to the simpler constitution 
of plants, the phenomena of irritability can be more easily 
studied. 
We now know that not merely such receptive centers as the 
pulvini of sensitive plants, the tentacular knobs of Drosera, 
and the irritant hairs of Dzone@a are irritable, and can also 
propagate a stimulus, but that stimuli can equally be trans- 
mitted from apparently passive parts. Thus a stimulus can be 
received and propagated from any part of a leaflet of Mimosa, 
or even from one of its stipules, from a cup gland on the 
petiolar base of Cassia nictitans and its allies, or from any 
part of the leaf surface of Drosera and Dion@a. What the 
substance or substances are which conduct such stimuli, and 
what the molecular changes are that are produced in these, 
we are still largely ignorant of, but that such primarily reside 
in, or are governed by protoplasm, appears to me to be still 
as true as when Sachs wrote: “We can at present form no 
idea why this change in the protoplasm occurs in consequence 
of a stimulus, and with what molecular changes it is con- 
nected ; it must suffice for us meanwhile to know that the 
externally perceptible effects of stimulations are caused by the 
change referred to in the protoplasm itself.” 
The specific irritability of such sensitive plants as JZmosa 
pudica, M. latispina, Schrankia uncinata and Desmodium 
canescens, gives to each different latent periods, times of 
