TRANSMISSION OF STIMULI. 49 



continues to be, a single individual, whose parts are only separated by perforated 

 sieve-like partitions. Every member of this community occupies a particular 

 compartment or cavity, and is governed by a central organ, the cell-nucleus; but 

 being linked to its fellows by connecting threads of protoplasm, a mutual under- 

 standing is thus established among them. 



The physical basis of such an understanding may in this manner be represented 

 with tolerable certainty. But it is extremely difficult to throw light upon the 

 process of this mutual intelligence, the actual method whereby the cell-nuclei 

 not only govern within their own narrow spheres, but also co-operate harmoniously 

 for the good of the whole. And yet the problem involved in this unanimity of 

 action, with a view to a systematic development of the plant in its entirety, is 

 of such extreme importance that we cannot evade it even if, in the endeavour 

 to solve it, we have to move altogether in the region of hypothesis. 



In every attempt at explanation of the kind we must, at all events, bear in 

 mind that the agreement in question, as well as the processes which take place 

 in pursuance of this agreement, such as the nutrition, growth, and the organization 

 of the entire plant, are reducible to the subtlest atomic agencies in the living 

 protoplasm. They may be resolved into the motion of minute particles, into 

 attractions and repulsions, oscillations and vibrations of atoms, and into re-arrange- 

 ments of the atomic groups called molecules. Again, these movements are the 

 result of the action of forces, especially of gravity, light, and heat. As regards 

 gravity and light, experiment shows, however, that, when acting on living proto- 

 plasm, they give rise to varying effects even under the same conditions; and this 

 fact, which will be discussed frequently later on, indicates that these forces are 

 at any rate only to be conceived as stimulative and not coercive, and that they 

 have no power to determine the kind of form. It is characteristic of the processes 

 set up by gravity and light, especially when they take place in the continuous 

 protoplasm of a great cell-community, that the coarser movements visible to the 

 naked eye are often manifested in members comparatively remote from the part 

 immediately affected by the stimulus. We cannot well represent this to ourselves 

 except by supposing that the stimulus, which is the cause of the movement, is 

 propagated through the threads of protoplasm from atom to atom, and from 

 nucleus to nucleus. But the great puzzle lies, as already remarked, in the circum- 

 stance that the atomic and molecular disturbances occasioned by such stimuli and 

 transmitted through the connecting filaments are not only different in the proto- 

 plasm of different kinds of plants, but even in the same plant they are of such 

 a nature, according to the temporary requirement, that each one of the aggregated 

 protoplasts in a community of cells undertakes the particular avocation which is 

 most useful to the whole, the effect of this joint labour conveying the impression 

 of the presence of a single governing power of definite design and of methodical 

 action. 



That a stimulus causes different occurrences in different species of plants, and, 

 more especially, that cell-communities arising from different egg-cells develop into 



