566 SUMMARY AND Chap. Xll 



the cotyledous of course remained expanded ; both 

 pots were now placed close together in the middle of 

 the room, and the cotyledons which had been exposed 

 to the sun, immediately began to close, while the 

 others opened ; so that the cotyledons in the two pots 

 moved in exactly opposite directions whilst exposed 

 to the same degree of light. 



We found that if seedlings, kept in a dark place, 

 were laterally illuminated by a small wax taper for 

 only two or three minutes at intervals of about three- 

 quarters of an hour, they all became bowed to the 

 point where the taper had been held. We felt much 

 surprised at this fact, and until we had read Wiesner's 

 observations, we attributed it to the after-effects of 

 the light; but he has shown that the same degree 

 of curvature in a plant may be induced in the 

 course of an hour by several interrupted illumina- 

 tions lastin'g altogether for 20 m., as by a continuous 

 illumination of 60 m. We believe that this ease, 

 as well as our own, may be explained by the ex- 

 citement from light being due not so much to its 

 actual amount, as to the difference in amount from 

 that previously received ; and in our case there were 

 repeated alternations from complete darkness to light. 

 In this, and in several of the above specified respects, 

 light seems to act on the tissues of plants, almost in 

 the same manner as it does on the nervous system 

 of animals. 



There is a much more striking analogy of the same 

 kind, in the sensitiveness to light being localised in 

 the tips of the cotyledons of Phalaris and Avena, and 

 in the upper part of the hypocotyls of Brassica and 

 Beta ; and in the transmission of some influence from 

 these upper to the lower parts, causing the latter to 

 bend towards the light. This influence is also trans- 



