194 



BIOLOGICAL LECTURES. 



ability this is due to the gradual passage of sap through the 

 contractile protoplasmic layer of each cell and the elastic recoil 

 of it and of the wall as additional liquid is extruded or absorbed. 



I now pass to Oxalis dendroides, a plant eminently suited for 

 investigations like the present, and which has yielded results 

 as interesting as they were unexpected. For a suppl)' of fine 

 specimens I am indebted to the kindness of Mr. G. Oliver of 

 the Washington Botanic Garden. It is an abundant weed in 

 Brazil and is often confounded with Oxalis scusitiva, that is 

 native from Persia to China. It is one of a series of nearly 

 related forms, some of which like the present have a simple 

 unbranched upright habit, while others incline to a proliferous 

 mode of growth. Though seldom seen outside botanic gardens, 

 it grows with the utmost readiness, fruits freely, and scatters its 

 seeds widely. Four noteworthy points can be readily demon- 

 strated on this species, though witnessed less perfectly in 

 others. These are: First, that the latent period varies accord- 

 ing to the age of the leaf; second, that a gradual propagation 

 of stimulus from base to apex or vice versa can be shown to 

 exist; third, that the rate of propagation of the contraction- 

 stimulus can be exactly measured ; and fourth, that the rate of 

 contraction and expansion of leaflets is quickest in young and 

 slowest in old leaves. 



If the tip of the blade of a terminal leaflet be stimulated 

 by a forceps snip, in an atmosphere whose temperature 

 is 28°- 32° C., it and the companion leaflet will fall 

 down through an angle of 40°-45° in about 20 seconds. 

 That irritation of a leaflet should excite to a rapid movement 

 not merely of one but of neighboring leaflets, suggests the idea 

 that at hast some of the cells of every leaflet can conduct a 

 stiviiiliis. This idea is entirely confirmed by many other 

 experimental results. If the leaflet that has been irritated be 

 part of an old leaf, the succeeding pairs of leaflets will close in 

 regular succession from apex to base with a time interval 

 between each pair of 2\ seconds. By this we mean the time 

 required for propagation of the shock from one pair of pulvini 

 to another pair below, and succeeding contraction of the living 

 protoplasm of the different cells in the path of the stimulus. 



