1 122 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



believe that there is no structure in plants more wonderful, so far as its 

 functions are concerned, than the tip of the radicle " ; and he also said it 

 acts like the brain of one of the lower animals. Since the publication 

 of these noteworthy remarks in 1880, various marvellous experiments 

 have been performed, especially by Mr. Francis Darwin, the results of 

 which do but strengthen and emphasise their truth. No less than nine 

 tropisms or responses, due to as many distinct stimuli, are incidentally 

 alluded to in this paper, but the author deals only with his work on 

 rheotropism, the response due to a current of water. The author writes : 

 1 In a recent paper * evidence was offered to show that the stimulus 

 giving the response known as rheotropism is perceived by the apical 

 millimetre of Zea Mays, L., and of Baphanus sativus, L. There was 

 evidence to show also that, in the root of the latter plant, the stimulus 

 was perceived throughout the elongating zone, and still more posteriorly, 

 to a distance even 15 mm. back of the apex, this being at least 10 mm. 

 beyond the limit of the elongating zone. This discovery was of such 

 interest that it seemed advisable to extend the research to other plants, 

 to ascertain whether the phenomena observed in Baphanus might be 

 found to be general. The results obtained in such an investigation are 

 reported in the present paper." The author describes his experiments 

 with several species, and in his summary and conclusion says : " As far 

 as we know, there is no reason in the nature of the case why the sensi- 

 tiveness of roots to stimuli should be confined to the region capable 

 of elongation. A grass stem receives and responds to the gravitation 

 stimulus when under ordinary conditions its growth has ceased ; the 

 cotyledon of Panicum is sensitive to light after growth has ceased ; and 

 the leaves of Mimosa and of numerous other plants continue to receive 

 and respond to stimuli long after growth has ended. The region of roots 

 shown in the foregoing pages to be sensitive to stimuli, though no 

 longer capable of elongation, may still grow in diameter. And it may be 

 said here that the assumption of the limitation of sensitiveness of roots 

 for certain stimuli to certain regions is not in all cases justified. 

 Czapek appears to have tenable ground for limiting the perception of 

 gravitation to the root apex. But that moisture stimulates only the 

 apex cannot be maintained until the elongating zone and the back part 

 of it are tested in the neutralisation of gravitation. So also with thermo- 

 tropism, aerotropism, and the like ; for aught we know, the stimuli for 

 these responses may be received beyond as well as in the elongating 

 zone. ... Of course there is pressure against the roots by the stream 

 of water. But thigmotropism (response to touch of rough surface) has 

 not been, and probably cannot be, demonstrated in ordinary roots by 

 ordinary means. If rheotropism is not thigmotropism, we do not know 

 what the nature of the stimulus is ; but until thigmotropism has been 

 demonstrated in roots, no claim can'be made for the identity of these tw r o 

 responses."-- R. I. L. 



* Newconibe, " The Rheotropism of Roots," Bot. Gar. xxxiii., 1K02, p. 117. 



