332 President's Address. 



If insolation bciuterruptedjust at the moment when the motion ceases, 

 in many cases the movement will sooner or later recommence and go 

 on normally. This may be observed in all kinds of movements, alike 

 in the motion of granules in the protoplasm between the chlorophyll- 

 bands of Spivorjyra, in the circulation in Tradmcanfia hair cells, 

 and in the rotation in Nitella. The condition of paralysis in the 

 cells of staminal hairs of Tradeseantia virginica, and in suitable 

 (short) cells of Nitella, may develop long before they are deco- 

 lorised. Careful study of the gradual cessation of movement in 

 intense light convinces one that the light at the place where it 

 reaches the protoplasm creates directly a hindrance to the movement 

 a fact explicable by the changes (just described) it brings about in 

 the substance of the ]Drotoplasm. As soon as light has operated suffi- 

 ciently long, one sees, e.g., in Nitdla, the movement slowing at the 

 insolated spot, and the streaming protoplasm in consequence aggre- 

 gates in large, often striated, masses about it. Then, especially if 

 the cell be long and the isolated area near its middle, as the current 

 cannot cross the insolated area, two currents frequently arise in one 

 cell, each following a course of its own, separated from the other by 

 the insolated part. 



These jAenomena are to be explained by the greater immobility of 

 the protoplasm which has been exposed to light. Because protoplasm 

 aggregates at the insolated area, it does not necessarily follow that 

 it is as it were drawn by the light, but only that in certain intensities 

 the motion of the protoplasm is slowed at the insolated area. And 

 so also the opposite effect, the aggregation of the protoplasm at the 

 shaded spots, is to be explained by the commencing immobility of 

 the insolated portion forcing the protoplasm into other channels. 

 The disposition of the moving masses in the cell shows only the 

 relative motility of the protoplasm in different, relatively darker 

 and lighter, places corresponding with the oxidation of its sub- 

 stance as infiuencd by light. 



{d) The membrane of the cell in intense Ugh t. — The cell wall exhibits 

 no very striking changes. At times, in more delicate Spirogijra 

 cells a slight swelling may be seen. If, however, delicate species of 

 Spirogyra, e.g., Sp. Weheri or species of Mesocarpus, be exposed to 

 light until death of the cells sets in, the isolated cells separate more 

 or less completely one from another, and in Sp. Wcberi the infolded 

 ends of the cells unfold. The changes in turgescence of the 

 insolated cells apj^ear to have nothing to do with this appearance ; 

 nor is any perceptible shortening or lengthening of the cell walls 

 apparent. But at the moment of separation a slight torsion of the 

 threads is observable, and possibly an inequality in tension between 

 the cuticle and the inner layers of the membrane, developing under 



