SENSORY MECHANISM OF PLANTS. 175 



Very naturally the first studies made in tliis subject attempted to 

 discover an arrangement in plants comparable to a simplified neuro- 

 Tiiuscular system of the animal. Expectations of this character were 

 of course bound to meet with signal disappointment ; a fact that should 

 have been apparent if the history and widely different purpose of the 

 animal and vegetal organism had been taken into consideration. 

 I'arallelisms between the reactions of plants and animals even to the 

 same class of stimulus are to be accepted with great caution. Thus it 

 has recently become apparent that the heliotropism of animals as in- 

 \estigated by Loeb is widely different from the heliotropism, or photo- 

 tropism, of plants both as to the features of light acting as stimuli in 

 the separate cases, and the general nature of the consequent reactions. 



Eecent papers by Nemec on the transmission of impulses in plants, 

 and the discussions of geotropism and the organs of equilibrium of 

 plants by JN'oll, Czapek and Haberlandt have awakened much interest 

 in the mechanism of irrito-motility of plants. 



ISTot fully appreciating the significance of the diffused and general- 

 ized forms of perception organs, much effort has been directed toward 

 fixing on specialized protoplastic tracts, with functions analogous to 

 nerves. The quest has not yet met with decided success in any single 

 instance. We have, however, arrived at the general conclusion that 

 the ectoplasmic layers of the protoplasts of peripheral cells function 

 as sensory organs, and that impulses are transmitted between the motor 

 and sensory zones by these layers and their interprotoplastic threads. 

 As to the nature of the impulse, one can only hazard a meaningless 

 guess that it may consist in a chain of chemical, catalytic or osmotic 

 disturbances. 



Two noteworthy attempts have been made to ascribe the function 

 of transmission of impulses to specially differentiated structures. The 

 first was by Haberlandt who dealt with the transmission of impulses in 



Fig. 4. Elongated Elements of Mimosa supposed by Haberlandt to transmit hy- 

 drostatic Impulses. 



Mimosa, the common 'sensitive plant' of the tropics, cultivated in con- 

 servatories. An impulse set up at the tip of a pinnule of one of these 

 plants is conducted through petioles, stems and branches to a distance 

 of a meter at a rate varying from 6 to 31 mm. per second. A study of 

 the structure of the plant reveals the presence of a connected series of 

 long tube-like cells in the fibro-vascular bundles, usually turgid, and 

 containing relatively small protoplasts. It is argued that impulses take 

 the form of hydrostatic disturbances communicated through the system 



