206 BIOLOGICAL LECTURES. 



mechanical, thermal, luminous, chemical and electrical stimuli, 

 and that the degree of contraction is proportioned to the rela- 

 tive molecular activity of the protoplasm and the strength or 

 continuity of the stimulus. 



Accepting this position, I venture to think that we can 

 harmonize many movements that appeared superfluous or dis- 

 tinct from each other. It may be asked, and indeed has 

 been asked by Darwin, Why are plants that are nyctitropjc 

 and parathermotropic often very sensitive to impact, though 

 apparently deriving no benefit from impact sensitivity ? We 

 reply that, being sensitive to, or irritated by, light, heat, or 

 cold, they must of necessity be also sensitive to impact, 

 even though deriving no benefit therefrom, since contraction- 

 sensitivity involves response to all forms of energy. But let 

 us be cautious in assuming that no benefit is got from a 

 certain movement unless such benefit is patent to us. Sachs 

 says about the sensitive plant, " So far as I am aware, no one 

 has attempted an explanation of the use of the irritability of 

 the leaves of Mimosa ; but I believe that I am able to afford 

 one ; for I have often had opportunities of observing that after 

 a severe hail-storm, when plants of the most various kinds 

 and even robust plants, close to my mimosas, before the 

 window or in the open have been clashed and broken by 

 the hail-stones, the mimosas, in spite of their delicate 

 structure, have come out quite uninjured ; a few minutes after 

 the rough weather they expanded their leaves again, entirely 

 unhurt." He might have added that not merely from hail, 

 but from beating winds and rain, the leaves are the better 

 protected ; as I have proved for Mimosa, Oxalis, Dcsmo- 

 dium, Ampliicarpcea, etc. As with animal contractile tissues, 

 then, every irritable plant is to a greater or less degree 

 irritable to all forms of stimuli. We derive now from this a 

 likely explanation of nyctitropic and parathermotropic move- 

 ments in plants. It is universally recognized that every 

 species has an optimum as well as a minimum and maximum 

 temperature relation. To return to O.ralis stricta again, the 

 optimum during the day is 24 26 C., but when it is exposed 

 to a steady heat-stimulus from the sun's rays of 3o-32 C., 



