1878.] TOWER OF MOVEMENT IN PLANTS.* 503 



cotyledons, in which the sleep of plants is to be observed in 

 its simplest form ; in the following spring he was trying to 

 discover what useful purpose these sleep-movements could 

 serve, and wrote to Sir Joseph Hooker (March 25th, 1878) : 



" I think we have proved that the sleep of plants is to 

 lessen the injury to the leaves from radiation. This has in- 

 terested me much, and has cost us great labor, as it has been 

 a problem since the time of Linnaeus. But we have killed or 

 badly injured a multitude of plants : N. B. Oxalis carnosa 

 was most valuable, but last night was killed." 



His letters of this period do not give any connected ac- 

 count of the progress of the work. The two following are 

 given as being characteristic of the author :] 



C. Darwin to W. Thiselton Dyer. 



Down, June 2, 1878. 



MY DEAR DYER, I remember saying that I should die a 

 disgraced man if I did not observe a seedling Cactus and 

 Cycas, and you have saved me from this horrible fate, as they 

 move splendidly and normally. But I have two questions to 

 ask : the Cycas observed was a huge seed in a broad and very 

 shallow pot with cocoa-nut fibre as I suppose. It was named 

 only Cycas. Was it Cycas pectinata ? I suppose that I can- 

 not be wrong in believing that what first appears above ground 

 is a true leaf, for I can see no stem or axis. Lastly, you may 

 remember that I said that we could not raise Opuntia nigri- 

 cans j now I must confess to a piece of stupidity ; one did 

 come up, but my gardener and self stared at it, and concluded 

 that .it could not be a seedling Opuntia, but now that I have 

 seen one of O. basilaris, I am sure it was ; I observed it only 

 casually, and saw movements, which makes me wish to ob- 

 serve carefully another. If you have any fruit, will Mr. 

 Lynch * be so kind as to send one more ? 



I am working away like a slave at radicles [roots] and at 



* Mr. R. I. Lynch, now Curator of the Botanic Garden at Cambridge- 

 was at this time in the Royal Gardens, Kew. 



