50 TRANSMISSION OF STIMULI. 



different forms, though under identical conditions and subjected to the same stimuU, 

 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 



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