50 TRANSMISSION OF STIMULI. 
different forms, though under identical conditions and subjected to the same stimuli, 
are phenomena which have parallels in the inanimate world. A different sound 
is produced by striking the key of a piano which is connected to an A-string from 
that resulting from the transmission of a similar impulse to an F-string; and the 
difference depends on a difference of structure and an inequality of tension in 
the strings. Again, solutions of the sulphate and of the hyposulphite of sodium 
in similar glass vessels are indistinguishable at sight, both being colourless and 
transparent. These solutions will preserve their liquid condition when cooled 
down gradually to below freezing-point if they are kept absolutely still; but the 
moment the vessels are touched and a vibration thereby transmitted to the contents, 
they freeze. Crystals are formed in the apparently identical liquids, but crystals 
of different kinds, Glauber’s salts in the one case, hyposulphite of sodium in the 
other. The variety of form depends simply on the sort of atoms, and on their 
number and mode of grouping. 
In a similar manner must be explained the variety of forms in many plant- 
species developed under the same conditions and affected by the same stimuli. 
Dozens of kinds of unicellular Desmids and Diatoms are often developed at the 
same time in a single drop of water in close proximity to one another. Although 
the protoplasm in the spores of these different species is absolutely identical to 
our vision, aided by the best microscopes, yet the mature cells exhibit a multiplicity 
of form which is quite astonishing to the observer on first inspection. One cell 
is semi-lunar, another cylindrical, a third stellate, a fourth lozenge-shaped, and 
a fifth acicular. In one specimen the cell-membrane is smooth, in another it is 
beaded; some are provided with siliceous coats, whilst others have flexible envelopes. 
The same thing holds good with respect to the vegetable structures, which are 
composed of myriads of cells, and develop into huge shrubs or tall trees. The 
protoplasm in the egg-cell of an oleander is produced close to that of a poplar on 
the same river-bank, and under exactly the same external conditions. The cells 
divide, and partition-walls are introduced in the proper direction in either case, 
according to a plan of structure which is adhered to with marvellous precision 
by the protoplasts engaged in the work of construction. In each species, stem, 
branches, foliage, and blossoms have invariably a particular form and arrangement, 
have the same colour and smell, and contain the same substances. How utterly 
different are the mature leaf, the opened flower, and ripe fruit of the oleander from 
the corresponding parts of a poplar. Yet both were nourished by the same earth, 
were surrounded by the same atmosphere, and encountered the same rays of sun- 
shine. We cannot otherwise explain it than by the supposition that, in a case 
like this, the difference of form in the perfected state is based upon a difference 
in the self-developing protoplasm, and that the atoms and molecules of this proto- 
plasm, which appears to us to be uniform, vary in kind, number, and grouping 
in the two species of plants. Consequently, we must assume that every vegetable 
organism, every species of plant that appears invariably in the same external 
form when mature, and develops according to an invariable plan, has a protoplasm 
