222 
J. Bretland Fartnef. 
When one conies to study those tissues that specially exhibit 
obviously irritable properties, one finds also strong evidence in 
favour of the association of a stimulus with a material change of 
the stimulated part, and also we learn that closely similar results 
may be produced in structures exhibiting widely diverse anatomical 
structure. Many tendrils are very irritable if touched in certain 
places, and the investigations of Haberlandt and others have shewn 
that in such cases special arrangements are often to be met with by 
which when they are touched, the protoplasm is deformed or 
squeezed. 
Thus the stimulus is mechanically given in the first instance, 
but its initial effect on the protoplasmic mechanism is to produce a 
molecular change that results finally in a movement or change of 
form. In some cases it seems certain that the change works by altering 
the surface membranes of the protoplasts, thereby influencing the 
distribution of osmotic pressures in the affected tissue. Such is 
pretty certainly the case in such a plant as the Venus’ Flytrap, 
when in consequence of an appropriate stimulus applied through 
the superficial hairs borne upon the upper surface of the leaves, the 
latter close up with such remarkable rapidity. The stimulus acts 
by causing a dimunition of turgor in the cells of the upper surface, 
and the now unbalanced turgid cells of the lower surface bring 
about the instantaneous closure of the leaf. 
Similarly, in multicellular structures, the change of position 
produced by the one-sided action of light or gravity appears to 
occur through the influencing of the turgescence of the cells on the 
stimulated side, but the precise effect that will happen depends 
entirely on the nature of the stimulated mechanism. This is proved 
by the fact that the effect may even become reversed, as when a 
positively heliotropic organ becomes negatively heliotropic as it 
becomes older—a striking example of this is seen in the ivy-leaved 
toad-flax, in which the flowering pedicils are positively heliotropic, 
but later on, when the fruit is setting they bend away from the 
light. This can mean nothing more than that the material 
stimulable structure has undergone a change—for it is of course of 
no use to invoke a mere teleological explanation of the result. The 
process has its use, as far as the species is concerned (in effecting 
the sowing of the seeds), but the action itself is as independent 
of personal utility as is the gravitational attraction of two lumps of 
matter for each other. But the heliotropic phenomena of uni¬ 
cellular hyphae of fungi shew that change in conditions of turgor is 
