AND IRRITABILITY OF SENSITIVE PLANTS 221 



Biophytum dendroides 

 The genus Biophytum includes some 20 species scattered through 

 the tropics of both hemispheres. B. dcndroides, indigenous to Brazil 

 and Peru, is readily grown in greenhouses where it flowers and fruits 

 very freely. Fifteen to twenty-five pairs of oblong leaflets are borne on 

 each of the pinnately compound leaves. A short, rather thick secondary 

 pulvinus is present at the base of each leaflet, and a well defined primary 

 pulvinus at the base of the petiole. A similar pulvinus is found at 

 the base of each of the long flower stalks. 



The night sleep of this plant is very striking. The leaflets drop 

 downward through an angle of 85-90 degrees, and the whole leaf falls 

 through 15-20 degrees. The upright flower stalks drop 70-80 degrees 

 (Plate LXIII, Fig. 16-17). The paraheliotropic response is equally pro- 

 nounced when the temperature, in bright light, reaches 29-30° C. 



The leaflets are likewise quite highly sensitive to mechanical, 

 chemical, thermal and electrical stimuli. Macfarlane (30 p. 194-195) 

 describes fully the effects of the first three of these forms of stimulation. 

 When a terminal leaflet is stimulated mechanically it and its neighbor fall 

 through an angle of 40-45 degrees, after a latent period of % second to 

 2 seconds, varying with the relative age of the leaflets. The stimulus 

 is propagated through the midrib, the pairs closing in regular succession 

 toward the base, with a time interval of 2}/^ seconds between each pair. 

 If a second stimulus is applied, the leaflets will fall through 20-25 de- 

 grees, and on the application of a third stimulus they fall through a small 

 angle. A fourth and even a fifth stimulus may cause a slight additional 

 drop. Summation action is here necessary to cause closure of the leaf- 

 lets." 



Macfarlane (30 p. 196) proved very conclusively that in Oxalis 

 dendroides, as in other sensitives, a stimulus is propagated more rapidly 

 in a centrifugal, than in a centripetal direction. "A particle of ice 

 placed on the pulvini of the middle pair, i. e., the tenth if there 19 

 pairs, will excite all the pairs above it within 15-17 seconds, but 21-23 

 seconds will elapse before the lowest pair in such a leaf as the tenth 

 from the apex-bud closes." 



Leaflets. The crystals here are confined only to the endodermal re- 

 gion of the vascular bundles, being arranged in fairly continuous lines. 

 In shape, the crystals resemble those of the Mimosas, but are less uni- 

 form in size and more irregular in shape than in the Mimosas. Many 

 of the crystals are six-angled and indicate a line across the middle. 

 (Plate LXIV, Fig. 20). 



