426 BOTANY [Chap. XI 



Letter 757 the diseases of plants would be well worth his study ; but I did 

 not know that it was written by Dr. Frank, 1 for whom I 

 entertain a high respect as a first-rate observer and experi- 

 mentiser, though for some unknown reason he has been a 

 good deal snubbed in Germany. I can give you one good 

 case of regrowth in plants, recently often observed by me, 

 though only externally, as I do not know enough of his- 

 tology to follow out details. It is the tip of the radicle of 

 a germinating common bean. The case is remarkable in 

 some respects, for the tip is sensitive to various stimuli, and 

 transmits an order, causing the upper part of the radicle to 

 bend. When the tip (for a length of about 1 mm.) is cut 

 transversely off, the radicle is not acted on by gravitation or 

 other irritants, such as contact, etc., etc., but a new tip is re- 

 generated in from two to four days, and then the radicle is 

 again acted on by gravitation, and will bend to the centre of 

 the earth. The tip of the radicle is a kind of brain to the 

 whole growing part of the radicle ! 2 



My observation will be published in about a week's time, 

 and I would have sent you the book, but I do not suppose 

 that there is anything else in the book which would interest 

 you. I am delighted that you have drawn attention to galls. 



1 Albert Bernhard Frank (1839- 1900) began his botanical career as 

 Curator of the University Herbarium, Leipzig, where he afterwards 

 became Privatdocent and finally " Ausserordentlicher Professor." In 

 1881 Frank was appointed Professor of Plant-Physiology in the Land- 

 wirthschaftliche Hochschule, Berlin. In 1899 he was appointed to the 

 Imperial Gesundheits-Amt in Berlin, and raised to the rank of 

 Regierungsrath. Frank is chiefly known for his work on " The Assimi- 

 lation of Free Nitrogen, etc.," and for his work on "The Diseases of 

 Plants" (Die Krankhciten der Pflanzen, 1880). It was his brilliant 

 researches on growth-curvature (Beitrage zur Pflatizen-physiologie, 1868, 

 and Die Naiiirlichcn wagerechte Richtung von PJlanzen-theilen, 1870) 

 which excited Darwin's admiration. 



2 We are indebted to Mr. Archer- Hind for the translation of the 

 following passage from Plato ( Timceus, 90A) : " The reason is every 

 man's guardian genius (dalfj-wv), and has its habitation in our brain ; it 

 is this that raises man (who is a plant, not of earth but of heaven) to an 

 erect posture, suspending the head and root of us from the heavens, 

 which are the birthplace of our soul, and keeping all the body upright." 

 On the perceptions of plants, see Nature, Nov. 14th, 1901 — a lecture 

 delivered at the Glasgow meeting of the British Association by Francis 

 Darwin. See also Bonitz, Index Aristotelicus, s.v. (pvrov. 



