ARY 17, 1923] 









ane 

TR roblem of the conduction of excitation in 
5 eecticahns is one that concerns both plant and 
animal physiologists, and any advances in our under- 
standing of conduction in either kingdom should be 
of common interest to all. Yet certain recent 
discoveries concerning excitatory conduction in plants 
have so far not become very widely known. 
_ The problem comprises essentially two questions: 
first, what is the nature of the excitation itself? and 
secondly, how is excitation at one point in an organ 
able to lead in turn to excitation at a neighbouring 
point? As to the first, there may perhaps be indica- 
ti that excitation is something fundamentally 
similar in all protoplasm; but as to the second, it 
“may well be that the link connecting the excitation 
of one point with that of the next is quite different 
in the case idf different organs. In one case the 
nature of the link seems now to be well established— 
namely, in»the case of species of the “ sensitive ’’ 
genus Mimosa, on which Dr. U. Ricca* has carried 
out a remarkable series of experiments. 
_ As is well known, the spread of excitation in these 
plants is revealed mainly by the fall of the main 
petiole of the doubly compound leaf, the forward 
movements of the secondary petioles, and the folding 
together upwards in pairs of the leaflets. These 
movements can be brought about by injuring a 
leaflet, and also by inflicting cuts or burns on the 
main stem of the plant, which may lead to the 
spread of excitation along the stem and out over 
several leaves. It is principally on this conduction 
in the stem that Ricca has experimented. . 
The starting-point of his work is the proof that, 
as maintained long ago by Dutrochet, the path of 
conduction is the wood and not the phloem or cortex. 
To establish this he has made use, not of the well- 
known Mimosa pudica, but of Mimosa Sepzazzinii, 
in which it is possible to remove completely, in a 
ring round the stem, the tissues external to the 
cambium, thus laying bare the wood. 
-Such ringing does not prevent the excitation from 
assing, as is shown by the closure of the leaflets 
in the leaves above the ringed zone, after a part of 
the stem below the ring has been stimulated by 
cauterisation. Conduction can therefore take place 
withoutcortex. Further, by removing onelongitudinal 
half of the stem and then, in the remaining half, 
“peed off the extra-cambial tissues from the wood, 
e has been able to investigate the effects of stimulat- 
ing the two separately. Stimulation of the strip of 
wood leads to movements in the leaves above, even 
after the pith has been scraped away, whereas 
_ stimulation of the strip of phloem and cortex does 
not. Since the latter are known not to be insensitive 
to stimulus, it follows that they must be unable 
alone to conduct the excitation effectively. 
f Next Ricca confirms the fact, already known, that 
- conduction can pass through a zone of the stem that 
has been completely killed by heat, and he also 
shows that even when a zone of 4-5 cm. is maintained 
_ ata temperature above 150° F. this does not prevent 
_ the supply of water to the leaves above, nor the 
conduction of excitation. Going further, he divides 
the stem transversely and inserts the cut ends into 
the expanded ends of a narrow glass tube 8 cm. long 
and 1 mm. in diameter. An earlier experiment with 
a wider tube (1916, “a,” p. 94) is less convincing. 
2 Ricea, U., “ Soluzione d’ un problema di fisiologia,’’ Nuovo Giorn. Bot. 
Ital. 23, 1916, “a.” 
“Solution d’un probléme de physiologie,”’ Archives italiennes de Biologie, 
65, 1916, “b.”” 
NO. 2781, VOL. 111] 
NAT URE 

4 
| 
! 
| 
} 
} 
' 
237 
The Conduction of Excitation in Mimosa. 
_Cauterisation of the stem below the tube was followed 
by closure of the leaflets above it; and if the stimulus 
'was strong, a greenish substance was seen to issue 
from the lower cut end, and slowly to spread up the 
tube. The time taken by the coloration to spread 
agreed roughly with the time apparently taken by 
the excitation to pass the tube (see schedules, Joc. cit., 
|p. 119 sq.). 
Already these results suggested that conduction 
takes place by the transference of a soluble stimulating 
substance excreted by the stimulated cells; for 
increase of permeability and excretion of liquid is 
known to accompany excitation in Mimosa and other 
plants. The final experiment in confirmation of this 
was the extraction of the substance by preparing in 
a small quantity of water a large number of transverse 
sections of stem. Other cut branches were then 
placed with their cut ends in the liquid thus obtained, 
and thereby excitation was found to be set up in 
them and to spread gradually up from the cut end 
towards the apex, as shown by the successive move- 
ments of their leaves. 
It seems clear then that conduction both in the 
glass tube and in the wood of the plant must be 
brought about by the movement of a stimulating 
substance with the water current. It cannot be 
due to pressure changes; first, because it is too slow 
(in one case 55 cm. in 1} hours: average values for 
M. pudica are 8-15 mm. per sec. in the petiole and 
2-3 mm. per sec. in the stem) ; and secondly, artificial 
changes in pressure of the water-supply to cut 
branches were not found to result in stimulation. 
In agreement with this, factors increasing transpira- 
tion and so accelerating the ascent of water in the 
stem were found to increase the rate of conduction. 
Still, it may remain uncertain whether movements 
of the water current alone can account for all cases 
of conduction in these plants, particularly for basipetal 
conduction in the leaves. i Mimosa Spezazzinit 
this takes place only with difficulty, and Ricca 
considers it due to the excretion of liquid from the 
stimulated region, which is then sucked away 
in both directions by neighbouring unstimulated 
tissues. In Mimosa pudica basipetal conduction 
takes place rapidly and easily. Possibly the activity 
of other living tissues along the conducting zone may 
in some cases be involved, even if it is not necessary 
for conduction in the stem. It is also desirable 
that the results should be confirmed by other workers 
in warm countries. 
Comparison may be made with the conduction of 
excitation in the cotyledon of a grass seedling, which 
also seems to involve a stimulating substance. In 
this organ, various stimuli, striking on the tip alone, 
bring about a responsive curvature in the elongating 
region below. The excitation conducted from tip 
to responding region can pass through a. layer of 
gelatin, after the tip has been cut off and stuck 
on again. It appears that Stark (loc. cit.) has 
extracted the stimulating substance concerned. An 
excitatory process capable of passing through gelatin 
has also been found by the present writer in roots. 
But in these cases the mechanism of conduction 
in the tissues is still obscure, and probably different 
from that found in Mimosa. It appears that conduc- 
tion may here take place in parenchymatous tissues, 
and it is checked by local application of anesthetics 
and other physiological agents. 
R. Snow. 
* Boysen- Jensen. Ber. d. D, Bot. Ges., 31, p.559. 1913. Paal, Jahrb. f. 
wiss. Bot. 58, 1918. Stark, Jahrb. f. wiss. Bot, 60, 1921. 
