214 STECKBECK— ON COMPARATIVE HISTOLOGY 



stained deeply in aniline blue. After this process the crystals stand out 

 clearly as white prisms in sharp contrast to the deep blue of the proto- 

 plasmic sacs. Between the adjoining crystal cells delicate stria tions 

 are observed. 



Mimosa albida 



The large genus Mimosa was referred to before as including species 

 that represent the climax of sensitivity in the Leguminosae. Mimosa 

 albida {M. sensitiva) is native to the tropics of the New World like the 

 great majority of the members of the genus. In the seedling stage, 

 the first leaves bear 3 pairs of leaflets. Professor Macfarlane (Lecture 

 Notes) demonstrated that the basal pair is less sensitive than the two 

 terminal pairs. After 4 shock stimuli had been applied, the two ter- 

 minal pairs closed, while the basal pair was closed to about half the 

 extent of the terminals. The leaves that are developed later are bi- 

 pinnately compound, with two leaflets to each leaf, and each leaflet 

 bearing two pairs of secondary leaflets. The leaflets of the terminal 

 pair are of equal size, and are obovate in shape. Of the basal pair, 

 the outer is of the same shape, but slightly smaller than those of the 

 terminal pair; the inner is very small and rudimentary. In the older 

 leaves the small secondary leaflet disappears entirely. There is a well- 

 defined primary pulvinus at the base of the petiole; secondary pulvini 

 are present at the bases of the leaf midribs, and a tertiary pulvinus 

 at the base of each secondary leaflet. 



In the nyctitropic movement the terminal pair of secondary leaflets 

 move upward back to back. The large basal secondary leaflet folds 

 over the outer terminal pair; the small basal secondary leaflet also 

 moves upward; the midribs converge and drop so as to make an angle 

 of about 75 or 80 degrees with the petiole which also drops 40 or 45 

 degrees. This fully closed condition is observed only in the younger 

 leaves. In the older leaves, the leaflets are closed ^ of their full extent. 



The paraheliotropic movement of the leaves is in the same direction, 

 and the same changes in position are noted as in the nyctitropic response. 

 The movement begins in bright sunlight when the temperature reaches 

 29° C. 



The leaves of M. albida are sensitive to mechanical, chemical and 

 thermal stimuli. When a terminal secondary leaflet is stimulated, 

 after a latent period of 1 to 13^ seconds, it rises, followed very soon by 

 its partner; in 28 seconds the basal pair moved upward, and in 3 seconds 

 longer the midribs converged; in 8 seconds after that the basal pair of 



