1 86 THE CAUSES OF SPECIFIC SHAPE 



cells of CJiara and Nitella stimuli may be propagated at a rate of i to 8 mm. 

 per sec., which is little less than the rates observed in cardiac muscle l . 

 Correlative stimuli appear to travel with extreme slowness -, xvhereas the 

 mechanical transmission in Mimosa may be as rapid as 15 mm. per sec., 

 while in the leaf of Dionaca a change of electrical potential may travel at 

 the rate of 200 mm. per sec. Even this is less than the rate of propagation 

 in animal nerves, which may amount to 30 metres per sec., although in 

 certain nerves stimuli travel more slowly. It is evident that no proper 

 co-ordination could be maintained between the various muscles employed 

 in particular movements unless the nerves conducted messages with such 

 rapidity as to bring them all to their destinations at practically the same 

 time. On the other hand, a slow transmission of stimuli is sufficient for the 

 correlation of growth in plants. In a colony of Volvox, however, rapid 

 transmission of stimuli is required to ensure the harmonious working of the 

 cilia of the individual cells. 



The slow conduction of vital stimuli in most plants is sufficient to show 

 that they are not transmitted by electrical agency, and the fact that a helio- 

 tropic stimulus is still able to travel along the seedling leaf of Avcna when 

 the vascular bundles have been cut across, helps to determine the conducting 

 channels but not the mode of transmission 3 . 



PART V 



HYPOTHESES OF ULTIMATE STRUCTURE AND HEREDITY 



SECTION 54. Theoretical. 



The determination of any factor in the production of specific shape 

 simply reveals an external stimulus to which the plant, organ, or cell responds 

 in a particular manner, and leaves unknown the structure, composition, and 

 properties of the protoplast upon which the power of reaction depends. All 

 attempts to explain the power of reaction of the protoplasm from its 

 structure and properties therefore rest upon a purely hypothetical basis, 

 and it must suffice to give here the essentials of the different theories that 

 have been put forward 4 . 



All hypotheses agree in supposing the protoplasm to be composed of minute 

 living particles (Darwin's pangens) which grow and reproduce themselves, and 

 which either directly or after union into larger groups unite to form the protoplasm. 



1 Ewart, Physics and Physiology of Protoplasmic Streaming in Plants, 1903, Sect. 47, p. 103. 



2 Townsend, Annals of Botany, 1897, Vol. XI, p. 509. 



3 The same applies to the conduction of wound-stimuli. Cf. Massart, La cicatrisation, 1898, p. 38. 



4 Details and literature are given by Delage, La structure du protoplasme et 1'heredite, 1895 ; 

 Wiesner, Elementarstructur, 1892 ; Hertwig, Zelle u. Gewebe, 1895, I, p. 267 ; 1898, II, p. 280. 



