AND IRRITABILITY OF SENSITIVE PLANTS 205 



Robinia Pseudacacia 



The black locust is native to the eastern United States, from Penn- 

 sylvania southward to Georgia, westward to Oklahoma, but has been 

 naturalized as far north as southern Canada. The imparipinnate leaves 

 of this tree show pronounced sensitivity to nyctitropic and parahelio- 

 tropic stimuli. At night the leaflets drop so as to be placed in an almost 

 vertical position. The movement under the effects of intense sunlight 

 is upward. In midsummer in bright sunlight when the temperature 

 reaches 27° C. or higher, the leaflets of the black locust are directed 

 toward the light at an angle to the vertical of 35 to 40 degrees. Later 

 in the day with a decrease in intensity of the sunlight and a gradual drop 

 in temperature the leaflets sink, until by 5 o'clock, as observed in a 

 number of cases, the leaflets were nearly or quite flat. Then there 

 followed a gradual sinking of the leaflets below the plane of the horizon, 

 by 7:30 or 8 o'clock they will have attained the night position, making 

 a very slight angle to the vertical. To mechanical or chemical stimuli, 

 Robinia is feebly responsive. If when the leaflets are expanded shock 

 stimuli be apphed to them, a slight drop will follow after a number of 

 stimuli have been given. Here then a summation of stimuli is necessary 

 to bring about response. 



Leaflets. Along the larger bundles of the leaflets two or three quite 

 regular rows of crystals are found. Along the small veins there is a 

 single line of crystals that is broken here and there. The crystals are 

 mainly of two kinds; one that is small, quadrangular in surface view, 

 seldom more than 7.6 microns long and 6.5 microns wide. These 

 crystals lie near the middle of cells of considerable size, each crystal 

 fining less than one third of the cavity of the cell in which it is contained. 

 The other type of crystal is hexagonal is outline, 15.2 microns long and 

 7.6 microns wide. Occasionally a crystal is noted in the chains along 

 the bundles that shows a line across its middle. The two types of 

 crystals are found in the same line with rather more of the hexagonal 

 prisms. The individual crystals of both kinds show quite wide varia- 

 tions in shape and size. The crystals are imbedded in protoplasmic 

 sheaths or envelopes. 



Pulvini. The secondary pulvinus of each leaflet shows rather a 

 wide cortical region in which crystals are distributed — relatively few 

 toward the epidermis and a considerable number just around the bundle, 

 in the endodermal region. The crystals are nearly all of the styloid or 



