660 KEPORT — 1891. 



Section D.— BIOLOGY. 



President of the Section — Francis Darwin, M.A., M.B., F.R.S. 



THUBSDAY, AUGUST 20. 

 The President delivered the following Address : — 



On Growth-curvatures in Plant?. 



A seedling plant, such as a young sunflower when growing in a state of nature,, 

 grows straight up towards the open sky, while its main root grows straight down 

 towards the centre of the earth. When it is artificially displaced, for instance, 

 by laying the flower-pot on its side, both root and stem execute certain curvaturefi 

 by which they reach the vertical once more. Curvatures such as these, whether 

 executed in relation to light, gravitation, or other influences, may be grouped 

 together as growth-curvatures, and it is with the history of our knowledge 

 on this subject that I shall be occupied to-day. I shall principally deal with 

 geotropic curvatures, or those executed in relation to gravitation, but the phe- 

 nomena in question form a natural group, and it will be necessary to refer to 

 heliotropism and, indeed, to other growth-curvatures. The history of the subject 

 divides into two branches, which it will be convenient to study separately. 



When a displaced apogeotropic organ curves so as to become once more 

 vertical, two distinct questions arise, which may be briefly expressed thus : — 



1. How does the plant recognise the vertical line ; how does it know where 

 the centre of the earth is ? 



2. In what way are the curvatures which bring it into the vertical line 

 executed ? 



The first is a question of irritability, the second of the mechanism of movement. 

 Sachs has well pointed out that these two very difl'erent questions bav© been con- 

 fused together.^ They should be kept as distinct as the kindred questions how. 

 by what nervous apparatus, does an animal perceive changes in the external 

 world ; and how, by what muscular machinery, does it move in relation to such 

 changes ? 



The history of our modern knowledge of geotropism may conveniently begin 

 with Hofmeister's researches, because in an account of his work some of the points 

 which re-occur in recent controversy are touched, and also because in studying his 

 work the necessity of dividing the subject into the two above-named headings. 

 Irritability and Mechanism will be more clearly perceived. 



In 1859 - Hofmeister published his researches on the effect of disturbance, such 

 as shaking or striking a turgescent shoot. This appears at first sight sufficiently 

 remote from the study of geotropism, but the facts published in this work were 

 tJie basis of the theory of geotropism formed by Hofmeister and accepted with 

 some modification by Sachs. When an upright, vigorously-growing, turgescent 

 shoot is struck at its base the upper end is made to curve violently towards the 



' Arheiten, ii. p. 282 (1879). 



' Hofmeister, Berichte d. k. Scicht. Get, d Tf'm., 1859. 



