268 Mr W. Gardiner, On the continuity [Nov. 13, 



examples, viz. by the sensitive plant, Mimosa padica. The com- 

 pound leaf of this plant is of that form known as bipinnate; 

 two or four secondary petioles spring from the main petiole of the 

 leaf; each such secondary petiole bearing from fifteen to twenty- 

 five pairs of leaflets, all the parts being connected the one 

 to the other by swollen joints or pulvini. In the daytime the 

 leaf assumes an expanded position, the main petiole forms an 

 acute angle with the upper half of the stem and the leaflets 

 are open, while at night the leaf falls and the leaflets close. 

 It has assumed its nocturnal or sleep position. If in the day- 

 time the leaf be touched or otherwise sufficiently stimulated, 

 it will fall and assume a position externally identical with that 

 of sleep. After some minutes it will rise and become expanded 

 only to fall again upon the application of a fresh stimulus, 

 but if the stimulus be too frequently repeated, it will soon 

 become insensible to the action and will no longer respond. If 

 placed for some time in the dark, it will execute spontaneous 

 movements, the leaf alternately rising and falling. Herein it 

 resembles the telegraph plant, except that the latter performs 

 its spontaneous movements irresjDective of the presence or absence 

 of light. In Mimosa the spontaneous periodicity is during the 

 daytime almost entirely concealed by the contractile part being- 

 very sensitive to the action of light. If it be not supplied with 

 oxygen it will lose its irritability and become asphyxiated, or if 

 exposed to the action of chloroform or ether, it will soon be 

 rendered insensible and will no longer react. This is the physio- 

 logical side of the question. 



Turning to the microscopical structure of such a pulvinus — 

 one notices that although three vascular bundles are apparent 

 on both the stem side and petiole side of the pulvinus — in that 

 organ itself, the three unite and furthermore become thinner, so 

 as to form a single axial bundle, which, as Sachs points out, lies 

 in the median plane of the organ and forms the neutral axis of 

 curvature. The whole bundle is flexible, slightly extensible, and 

 but little lignified. It is surrounded by a succulent mass of 

 parenchyma, consisting of roundish cells of about eight layers thick 

 covered by a badly differentiated uncuticularised epidermis from 

 which stomata are absent. The parenchyma cells immediately 

 under the epidermis are packed close together with no inter- 

 cellular spaces, but as one approaches the bundle, spaces of larger 

 and larger size become apparent, all of which are in communi- 

 cation one with the other. The cells of the under or more 

 bending side are thin walled — the upper are thick. All the 

 cells are very conspicuously pitted, especially on their longitu- 

 dinal walls, and they each contain at least one nucleus and a 

 large drop or drops of tannin. This is the morphological side. 



