14 RELATIONS OF PLANTS TO MECHANICAL FORCES 



affect the other leaves by transmission, use a burning match or 

 heated rod instead of the forceps to irritate the pinnule at the be- 

 ginning of the experiment. Make similar measurements with 

 Biophytum. Repeat both experiments and from the data thus 

 secured make out the average rate of transmission. Does the 

 impulse travel at the same rate in the direction of the root and 

 toward the apex of the shoot ? The time elapsing between the 

 reception of the stimulus, and the reaction includes also the 

 period necessary for the stimulus to be converted into a different 

 kind of molecular motion which traverses the tissues and sets 

 free the specific energy of the reacting mechanism. The impulse 

 will be found to travel at the rate of 8-20 mm. per second in 

 Mimosa, and 1—3 mm. per second in Biophytum. 



20. The Structure and Action of the Motor Organs. Cut trans- 

 verse and longitudinal sections of the pulvini at the bases of 

 the petioles and petiolules in Mimosa and Biophytum and examine 

 their structure with magnification of 400 or 600 diameters. The 

 chief features will be found to be a central cord- of fibrovascular 

 tissue, encased with a collenchymatous sheath. Outside of this 

 is a cylindrical mass of highly turgid parenchymatous tissue, 

 which is under such tension that the sections curl when placed on 

 the glass slip for examination. The communication of the im- 

 pulse to the pulvinus probably causes a contraction of the proto- 

 plasm of the cortical cells of the lower side of the pulvinus sim- 

 ilar to that exhibited by the amoeba (16) and allows some of 

 the water in the cell to pass out into the intercellular spaces. 

 This reduces the size of the cells concerned, and shortens that 

 side of the pulvinus, thus causing a movement. The central 

 strand of the pulvinus behaves like a thin- rod of flexible steel 

 sheathed in gutta percha. 



In Mimosa the shortening of the lower side of the pulvinus 

 allows the leaf to drop in response to its own weight, in addition 

 to the pressure of the opposite side of the organ. Fasten a plant 

 in an inverted position and when the leaves are normally ex- 

 panded, apply a stimulus to the tips of a leaflet and compare the 



