in Plant Cytology. 1^5 



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 oi Drosera, 

 and the irritant hairs of Dio7i(sa 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 Dioncea. 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 w4th 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 Mimosa 

 pudica, M. latispina, Schrankia tmcinata and Desmodium 

 canescens, gives to each different latent periods, times of 



