1898] RECENT PliOGRESS IN ROOT-PHYSIOLOGY 883 



of gravity and the reaction which it evokes take place in pre- 

 cisely the same part of the organ. Is the perception of gravity 

 spread over the entire growing (and at the same time potentially 

 curving) region of the root, or is it limited to any particular spot ? 



The first light which was cast upon this subject was due to the 

 observations of Ciesielski. He noticed, that when he removed the 

 tips of the roots of certain seedlings, tliey never curved from the 

 horizontal to the vertical position as did the unmutilated organs. 

 As soon, however, as the tip had become regenerated, the normal 

 geotropic activity was once more plainly visible. The root-tip, 

 Ciesielski therefore concluded, was an important and necessary 

 factor in bringing about the geotropic curvature. Charles Darwin^ 

 took up these experiments where Ciesielski had left them in 1872, 

 and greatly extended and elaborated them. The important con- 

 clusion to which they led him was, that the root-apex is the point 

 at which the geotropic stimulation is first received, whilst the zone 

 of growth which lies just behind the apex is the region which carries 

 out the reaction. 



These experiments and views were much criticised, and led to 

 numerous discussions. Some of the leading vegetable physiologists, 

 and among them Sachs, altogether rejected Darwin's interpretation. 

 In his " Lectures on Plant Physiology " Sachs writes, " In such 

 experiments with roots not only is great precaution necessary, but 

 also the experience of years and an extensive knowledge of vegetable 

 physiology, to avoid falling into errors, as did Charles Darwin and 

 his son Prancis, who, on the basis of experiments which were 

 unskilfully made and improperly explained, came to the conclusion, 

 as wonderful as it was sensational, that the growing-point of the 

 root, like the brain of an animal, dominates the various movements 

 in the root." It must be admitted that these adverse criticisms were 

 entirely justified, since the experimental proofs which were offered 

 by Ciesielski and Darwin were far from conclusive. 



So serious an operation as the ' decapitation ' of the root might 

 well be supposed to lead to changes in the irritability of the organ. 

 The recent work of Rothert upon Heliotropism (1894) has taught 

 us that removal of the apex of the cotyledon in certain Gramineae 

 temporarily (1-2 days) paralyses their perceptive powers for light, 

 notwithstanding the fact that the sub-apical parts of the cotyledon 

 are also sensitive to light in the unwounded plant. Facts of this 

 nature show us that there were good grounds for those who raised 

 objections to Darwin's conclusions. 



Matters remained in this highly unsatisfactory condition until 

 quite lately, when Dr P. Czapek undertook a comprehensive series of 

 researches on geotropism. The first-fruits of the work were com- 



1 Ch. Darwin " The Power of Movement in Plants " 1880. 



