July 15, 1875] 



NATURE' 



207 



volume, one which would alone have established the repu- 

 tation of any other author, and which will go far to 

 redeem our country from the charge of sterility in physio- 

 logical work. Much attention has been called recently to 

 the singular subject of ".carnivorous plants ; " we have had 

 records of useful original work from several quarters in 

 England, the Continent, and America, together with 

 much that has been superficial and worthless ; and even 

 the newspapers have discussed the anti-vegetarian habits 

 of some vegetables in the light, airy, and philistine 

 manner in which they are wont to approach " mere scien- 

 tific" subjects. During the whole of this time, foi the 

 last fifteen years, Mr. Darwin has been steadily and 

 quietly at work, collecting materials and recording long 

 series of observations ; and now at length has given us 

 their results, completely and finally settling some of the 

 points that have been most in controversy, and raising 

 others which suggest conclusions that will take by surprise 

 even those whose minds have been most open to deviate 

 from the old and narrow paths. 



Rather more than one-half of the volume is devoted to 

 the most abundant and readily obtainable of these preda- 

 tory plants, the common Sundew, Drosera rottmdifolia j 

 and an epitome of this portion must be first placed before 

 our readers. 



Commencing with a description of the well-known 

 leaves and their glandular appendages, or "tentacles," 

 as he terms them, Mr. Darwin has arrived at the conclu- 

 sion that these latter most probably existed primordially 

 as glandular hairs or mere epidermal formations (tri- 

 chomes), and that their upper part should still be so con- 

 sidered ; but that their lower portion, which alone is 

 capable of movement, consists of a prolongation of the 

 leaf ; the spiral vessels being extended from this to the 

 uppermost part. One point which seems to be clearly 

 established is, that it is not sufficient that the substance 

 which excites the movements of the tentacles should 

 merely rest on the viscid fluid excreted from the glands ; 

 it must be in actual contact with the gland itself. A state- 

 ment made by several previous observers (including Prof. 

 Asa Gray on the authority of Mr.' Darwin's earlier obser- 

 vations, and the present writer) — that inorganic substances 

 are almost or entirely without effect in producing 

 movement— must now be modified. Although the 

 effect is much less considerable, and the substance 

 is soon released from the embrace of the tentacles ; 

 yet such bodies as minute particles of glass un- 

 doubtedly possess the power of irritation. While it is 

 the glands or knobs at the extremities of the tentacles, 

 and a very small part of the upper portion of the 

 pedicels, which alone are sensitive or irritable, the actual 

 inflection takes place only in the lowermost portion of the 

 pedicel, causing a bending of the tentacle ; and the 

 irritation is conducted from the tentacle actually excited 

 to the neighbouring ones, or to all those on the leaf, in 

 such a manner as to cause them to bend towards the 

 object which produces the excitement. One of the most 

 striking of the [series of observations here recorded is 

 that which describes the affixing of exciting particles on 

 glands at two different portions of a leaf of Drosera^ the 

 result being that all the tentacles near each of these two 

 points were directed towards them, "so that two 

 wheels were formed on the disc of the same leaf, 



the pedicels of the tentacles forming the spokes, and 

 the glands united in a mass " over the irritated tentacle 

 which represented the axle ; the precision with which 

 each tentacle pointed to the irritating particle was 

 wonderful. What makes this result the more extraor- 

 dinary is that " some of the tentacles on the disc, which 

 would have been directed to the centre had the leaf been 

 immersed in an exciting fluid (as in Fig. i), were now 

 inflected in an exactly opposite direction, viz., towards 

 the circumference. These tentacles, therefore, had devi- 

 ated as much as 180° from the direction which they would 

 have assumed if their own glands had been stimulated, 

 and which may be considered as the normal one." As 

 the author remarks, "we might imagine that we were 

 looking at a lowly organised animal seizing prey with its 

 arms." Indeed, the whole description of Mr. Dar- 

 win's researches after the tissue that conducts this 

 irritation reminds one of experiments on the motor 

 and sensitive nerves of animals ; and we commend 

 the subject to the serious attention of the Royal Com- 

 mission now sitting to investigate the subject of vivi- 

 section. Mr. Darwin compares this movement to the 

 curvature displayed by many tendrils towards the side 

 which is touched j but the comparison appears to us to 

 fail, from the fact that the movement of tendrils is a 

 function of growth, they being sensitive to contact or 

 pressure only so long as they are in a growing state; 

 which is not the case with the tentacles of Drosera. One 

 of the most extraordinary of the statements made by trust- 

 worthy observers with regard to the sensitiveness of these 

 tentacles is not, however, confirmed by Mr. Darwin. Mrs. 

 Treat {American Naturalist, Dec. 1873) asserts that 

 when a living fly was pinned at a distance of half an inch 

 from the leaves of the American species D. filiformis, the 

 leaves bent towards it and reached it in an hour and twenty 

 minutes, a phenomenon inexpHcable on any theory which 

 would account for the transmission of the irritation from 

 one tentacle to another. Mr. Darwin states, on the contrary, 

 that when pieces of raw meat were stuck on needles and 

 fixed as close as possible to the leaves, but without actual 

 contact, no effect whatever was produced. The minuteness 

 of the solid particles which produced sensible inflection 

 was a matter of great surprise. Particles perfectly inap- 

 preciable by the most sensitive parts of the human body, 

 as the tip of the tongue— a fragment of cotton weighing 

 s^U, and of hair weighing ^gj^o of a grain— caused the 

 tentacles with which they were in contact to bend. Our 

 author remarks that " it is extremely doubtful whether 

 any nerve in the human body, even if in an inflamed con- 

 dition, would be in any way affected by such a particle 

 supported in a dense fluid, and slowly brought into con- 

 tact with the nerve ; yet the cells of the glands of Drosera 

 are thus excited to transmit a motor impulse to a distant 

 point, inducing movement ; " and he adds justly, that 

 " hardly any more remarkable fact than this has been 

 observed in the vegetable kingdom." The only substance 

 which appears to be altogether without effect in producing 

 inflection is drops of rain-water ; a singular exception 

 paralleled by the case of certain climbing plants whose 

 excessively sensitive tendrils are irritable to every sort ot 

 object which touches them except rain-drops. 



The inflection of the base of the tentacle is accom- 

 panied by a change in the molecular condition of the 



