542 



NATURE 



[September 27, 1900 



contain water, and that the current travels in the lumina and 

 not in the walls of the vessels. 



Now as to the force by which the water of the transpiration- 

 current is raised from the roots to the topmost leaf of a lofty 

 tree. From the point of view that the water travels in the 

 substance of the walls the necessary force need not be great, 

 and would be amply provided by the transpiration of the leaves, 

 inasmuch as the weight of the water raised would be supported 

 by the force of imbibition of the walls. From the point of view 

 that the water travels in the lumina, the force required to raise 

 and support such long columns of water must be considerable. 

 Dismissing at once as quite inadequate such purely physical 

 theories as those of capillarity and gas-pressure, there remain 

 two theories as to the nature of this force which resemble each 

 other in being essentially vitalistic, but differ in that the one 

 involves pressure from below, the other suction from above. In 

 the one, suggested by Godlewski and by Westermaier (1884), 

 the cells of the medullary rays and of the wood-parenchyma are 

 supposed to absorb liquid from the vascular tissue at one level 

 and force it back again by a vital act at a higher level : this 

 theory was disposed of by the fact that the transpiration-current 

 can be maintained through a considerable length of a stem 

 killed by heat or by poison. In the other, suggested by Dixon 

 and Joly (1895-99), and also by Askenasy {1895-96), it is 

 assumed that there are, in the trunk of a transpiring tree, con- 

 tinuous columns of water which are in a state of tensile stress, 

 the tension being set up by the vital transpiratory activity of the 

 leaves. Some idea of the enormous tension thus assumed is 

 given by the following simple calculation relating to a tree 120 

 feet high. Not only has the liquid to be raised to this height, 

 but in its passage upwards a resistance calculated to be equal to 

 about five times the height of the tree has to be overcome. 

 Hence the transpiration-force in such a tree must at least equal 

 the weight of a column of water 720 feet in height ; that is, a 

 pressure of about twenty-four atmospheres, or 360 lb. to the 

 square inch. But there is no evidence to prove that a tension 

 of anything like twenty atmospheres exists, as a matter of fact, 

 in a transpiring tree ; on the contrary, such observations as 

 exist {e.g. those of Hales and of Boehm) indicate much lower 

 tensions. Under these circumstances we must regretfully 

 confess that yet one more century has closed without bringing 

 the solution of the secular problem of the ascent of the sap. 



The nineteenth century has been, fortunately, rather more 

 fertile in discovery concerning the movements and irritability 

 of plants. But it is surprising how much knowledge on these 

 points had been accumulated by the beginning of the century : 

 the facts of plant movement, such as the curvatures due to the 

 action of light, the sleep-movements of leaves and flowers, the 

 contact-movements of the leaves of the sensitives, were all 

 familiar. The nineteenth century opened, then, with a con- 

 siderable store of facts ; but what was lacking was an inter- 

 pretation of them ; and whilst it has largely added to the 

 store, its most important work has been done in the direction of 

 explanation. 



The first event of importance was the discovery by Knight, in 

 1806, of the fact that the stems and roots of plants are irritable 

 to the action of gravity and respond to it by assuming definite 

 directions of growth. Many years later the term " geotropism " 

 was introduced by Frank (1868) to designate the phenomena of 

 growth as affected by gravity, and at the same time Frank an- 

 nounced the important discovery that dorsiventral members, 

 such as leaves, behave quite differently from radial members, 

 such as stems and roots, in that they are diageotropic 



It was a long time before the irritabiUty of plants to the action 

 of light was recognised. Chiefly on the authority of de Candolle 

 (to whom we owe the term " heliotropism '"), heliotropic curva- 

 ture was accounted for by assuming that the one side received 

 less light than the other, and therefore grew the more rapidly. 

 But the researches of Sachs (1873) and Muller-Thurgau (1876) 

 have made it clear that the direction of the incident rays is the 

 important point, and that a radial stem, obliquely illuminated, 

 is stimulated to curve until its long axis coincides with the inci- 

 dent rays. Moreover, the discovery by Knight (1812) of nega- 

 tive heliotropism in the tendrils of Vitis and Ampelopsis really 

 put the CandoUean theory quite out of court ; and further 

 evidence that heliotropic movements are a response to the 

 stimulus of the incident rays of light is afforded by Frank's 

 discovery of the diaheliotropism of dorsiventral members; 

 ■ The question of the localisation of irritability has received a 

 good deal of attention. The fact that the under surface of the 



NO. 16 1 3, VOL. 62] 



pulvinus of Mimosa pudica is alone sensitive to contact was 

 ascertained by Burnett and Mayo in 1827 ; and shortly after 

 (1834) Curtis discovered the sensitiveness of the hairs on the 

 upper surface of the leaf of Dionaea. After a long period of 

 neglect the subject was taken up by Darwin. The irritability 

 of tendrils to contact had been discovered by Mohl in 1827 ; 

 but it was Darwin who ascertained, in 1865, that it is confined 

 to the concavity near the tip. In 1875 Darwin found that the 

 irritability of the tentacles of Drosera is localised in the terminal 

 gland ; and followed this up, in 1880, by asserting that the 

 sensitiveness of the root is localised in the tip, which acts like a 

 brain. This assertion led to a great deal of controversy, but 

 the researches of Pfeffer and Czapek (1894) have finally estab- 

 lished the correctness of Darwin's conclusion. It is interesting 

 to recall that Erasmus Darwin had suggested the possible 

 existence of a brain in plants in his " Phytologia " (1800). But 

 the word "brain" is misleading, inasmuch as it might imply 

 sensation and consciousness : it would be more accurate to speak 

 of centres of ganglionic activity. However, the fact remains 

 that there exist in plants irritable centres which not only receive 

 stimuli but transmit impulses to those parts by which the con- 

 sequent movement is effected. The transmission of stimuli has 

 been found in the case of Mimosa pudica to be due to the 

 propagation of a disturbance of hydrostatic equilibrium along a 

 special tissue ; in other cases, where the distance to be traversed 

 is small, it is probably effected by means of that continuity of 

 the protoplasm to which I have already alluded. 



Finally, as regards the mechanism of these movements, we 

 find Senebier and Rudolphi, the earliest writers on the subject 

 in the nineteenth century, asserting, as if against some accepted 

 view, that there is no structure in a plant comparable with the 

 muscle of an animal. Rudolphi (1807) suggested, as an alter- 

 native, that the position of a mobile leaf is determined by the 

 "turgor vitalis" of the pulvinus, and thus anticipated the 

 modern theory of the mechanism. But he gives no explana- 

 tion of what he means by "turgor " ; and the term is frequently 

 used by writers in the first half of the century in the same vague 

 way. Some progress was made in consequence of the discovery 

 of osmosis by Dutrochet (1828), and more especially by his 

 observation (1837) that the movements of Mimosa are dependent 

 on the presence of oxygen, and are therefore vital. But it was 

 not, and could not be, until the existence of living protoplasm 

 in the cells of plants was realised, and the movements of free- 

 swimming organisms and naked reproductive cells had become 

 more familiar, that the true nature of the mechanism began to 

 be understood ; and then we find Cohn saying, as long ago as 

 i860, that "the living protoplasmic substance is the essentially 

 contractile portion of the cell." This statement may, perhaps, 

 seem to put the case too bluntly, and to savour too much of 

 animal analogy ; but the study of the conditions of turgidity has 

 shown more and more clearly that the protoplasm is the pre- 

 dominant factor. The protoplasm of plant-cells is undoubtedly 

 capable of rapid molecular changes, which alter its physical 

 properties, more particularly its permeability to the cell-sap. It 

 may be that these changes cannot be directly compared with 

 those going on in animal muscle ; but if we use the term " con- 

 tractility " in its wider sense, as indicating a general property of 

 which muscular contraction is a special case, then Cohn's state- 

 ment is fully justified. This is borne out by the observations of 

 Sir J. Burdon-Sanderson (1882-88) on the electrical changes 

 taking place in the stimulated leaf of Dionaea, and by Kunkel's 

 (1878J corresponding observations on Mimosa pudica : in both 

 cases the electrical changes were found to be essentially the 

 same as those observable on the stimulation of muscle. We 

 find, then, that the advances in Physiology, like those in 

 Anatomy, teach the essential unity of life in all living things, 

 whether we call them animals or plants. 



With this in our minds we may go on to consider in conclu- 

 sion, and very briefly, that department of physiological study 

 which is known as the Bionomics of CEcology of plants. In the 

 earlier part of the century this subject was studied more 

 especially with regard to the distribution of plants, and their 

 relation to soil and climate ; but since the publication 

 of the "Origin of Species" the purview has been greatly ex- 

 tended. It then became necessary to study the relation of plants, 

 not only to inorganic conditions, but to each other and to 

 animals ; in a word, to study all the adaptations of the plant 

 with reference to the struggle for existence. The result has 

 been the accumulation of a vast amount of most interesting 

 information. For instance, we are now fairly well acquainted 



