4IO 



NATURE 



[August 27, 1891 



came about that heliotropism was discassed, for instance, in 

 Sachs's " Text-book," edit. 4, 1874, under the same heading 

 as the influence of light on reotilinear grovvth. 



Shortly afterwards, in 1876, a pupil of Sach? — Muller-Thurgau 

 — published (" Flora") a research carried out in the Wlirzburg 

 Laboratory, which is of some importance. In the introductory 

 remarks he wrote : — " It has been hitherto supposed that helio- 

 tropic curvatures depend on a difference in intensity of illumina- 

 tion on the two sides. Sachs came to a different opinion in his 

 work on geotropism : he found himself compelled to believe 

 that in h;;liotropic, just ai in geotropic curvatures, it is not a 

 question of different intensities on opposite sides, but rather that 

 heliotropic effect depends on the direction of the light." ' 



Muller's research gave weight to this union of geo- and helio- 

 tropic effects by showing a number of resemblances in ihe 

 manner and form of the two curvatures. Again, when it was 

 found -' that apheliotropic organs are influenced by light and 

 darkness in precisely the same manner as positively heliotropic 

 ones, it became clear that the mechanical explanation of De Can- 

 dolle was untenable for negatively heliotropic organs, it might 

 still no doubt be upheld for positively heliotropic organs, but, 

 as a matter of fact, it was not so upheld. There was a tendency 

 to unify our view of growth-curvatures, and the union of the 

 two forms of heliotropism gave strength to the movement. Nor 

 was this all ; when it became clear that light did not produce 

 heliotropic curvatures by direct mechanical effect, it was natural 

 to remember that gravitation has none either ; we cannot point 

 to any reason (except the crudest ones) why the lower side of a 

 horizontal stem, or the upper side of a horizontal root, should 

 grow the faster for the direct effects of gravitation. That being 

 so, light and gravitation could be clashed together as external 

 agencies acting, not directly, but in some unknown indirect 

 manner. I do not imply that such a result followed immediately, 

 but that the line of research above alluded to helped in some 

 degree to lead the way to a belief in growth-curvatures as 

 phenomena of irritability. 



When my father was writing our book, " The Power of 

 Movement in Plants" (1880), in which he adopted to the fullest 

 extent a belief that growth-curvatures are phenomena of irrita- 

 bility, the only modern statement of such a view which he 

 could find was in a passage by Sachs [Arbeiten, ii., 1879, p. 

 282), where he writes that "The living material of plants is 

 internally differentiated in such a way that different parts are 

 supplied with specific energies resembling those of the sensory- 

 nerves [Sinnesnerven) of animals. Anisotropy in plants fulfils 

 the same purpose as do sense-perceptions in animals." 



The idea of irritability as applied to growth-curvatures is 

 expressed with sufficient clearness in "The Power of Move- 

 ment." Thus, for the case of geotropi.-m we wrote (p. 521) : — 

 " Different parts or organs on the same plant, and the same 

 part in different species, are thus excited to act in a widely 

 different manner. We can see no reason why the attraction of 

 gravity should directly modify the slate of turgescence and sub- 

 sequent growth of one part on the upper side, and of another 

 part on the lower side. We are therefore led to infer that both 

 geotropic, apogeotropic, and diageotropic movements, the pur- 

 pose of which we can generally understand, have been acquired 

 for the advantage of the plant by the modification of the ever- 

 present movement of circumnutation. This, however, implies 

 that gravitation produces some effect on the young tissues suf- 

 ficient to serve as a guide to the plant." A similar view is given 

 for heliotropism. It should be noted that the essence of the 

 view — namely, that light and gravitation act as guides or land- 

 marks by which the plant can direct itself— can be held without 

 a belief in circumnutation. 



In Pfeffer's admirable " Pflanzenphysiologie," 1881, the con- 

 ception of stimulus and reaction is fully given, and is applied, 

 among other cases, to that of heliotropism and geotropism. 

 Pfeffer states clearly, and without reserve or obscurity, the view 

 that light and gravitation act as stimuli or releasing forces, in 

 manners decided by the organization of the plant. Pfeffer 

 seems to me to be the first writer who has treated the subject 

 fully and consistently. 



In Sachs's " Vorlesungen " {1882), a view similar to that 

 briefly sketched in his paper of 1879 is upheld. Geotropism 



' In his "Vorlesungen," p. 854, Sachs states that he wrote Muller- 

 1 hurgau s introduction. 



^ Schmitz, Linniea, 184:5 ; Muller-Thurgau ("Flora," 1876); F. Darwin, 

 Sachs's Arbeiteti, 1880. The two latter researches were carried out under 

 the direction of Sachs in his laboratory, 



NO. 1139, VOL. 44] 



and heliotropism aie described as Reizerscheimmgen, i.e. phe- 

 nomena of stimulation. The phenomena in question are de- 

 scribed under the heading Anisotropy, a word which expresses, 

 according to Sachs (p. 855), " the fact that different organs of 

 a plant under the influence of the same external forces assume 

 the most varied directions of growth." In another passaee 

 (P- 859) he states that the anisotropy of the different organs " is 

 nothing else than the expression of their different irritability to 

 the influence of gravity [and] light, &c." 



Vines ("Physiology of Plants"), who has recently (1886) 

 summarized the evidence on growth-curvatures, and whose 

 researches on kindred subjects entitle his opinion to respect, 

 accepts fully the view that gravitation, light, &c., act as stimuli, 



It is not necessary to trace the subject fuither, the views 

 under discussion being now well-recognized canons of vegetable 

 physiology. 



I cannot, however, omit to mention Pfeffer's {Tiibing.n. 

 Untersuchiingen, vol. i. ) brilliant researches on the chemotaxis 

 (irritability to certain reagents) of low organisms, such as 

 antherozoids and bacteria. To take a single instance, Pfeffer 

 showed that the antherozoids, in responding to the effect of 

 malic acid, follow precisely the same law that in animals corre- 

 lates the strength of stimulus and amount of effect. This result, 

 although it has no direct connection with growth-curvatures, ^s 

 nevertheless of the highest importance in connection with the 

 general question of vegetable irritability. 



Nor can I omit to menaon the ingenious reasoning by which 

 Noll (Sachs's Arheiten, vol. ii. p, 466) localized the seat of irri- 

 tability in a vegetable cell. He points out how in acellular 

 plants, such as Caulerpa or Derbesia, the flowing protoplasm 

 may travel from positively geotropic root to apogeotropic stem, 

 and he argues from this that the motile endoplasm cannot be 

 the seat of specific irritability. The flowing plasma, which is 

 always changing its position with regard to external forces, must 

 be as fully incapacitated from responding to them as though 

 the plant were turning on a klinostat. It follows from this that 

 it must be the stationary ectoplasm which perceives external 

 change. From a different point of view, this is what we should 

 expect — we should naturally suppose that the part which regu- 

 lates the giowth of the membrane, and therefore the curvature 

 of the cell, shoidd be the irritable constituent of the cell 

 contents. 



In attempting to trace the history of the establishment of 

 growth-curvatures as phenomena of irritability, I have been 

 forced to confine myself to a slight sketch. I have found it im- 

 possilile to give a full account of the course of research on the 

 subject. I have given an account of some of the halting-places 

 in the journey of thought, but not to the manner in which belief 

 has travelled from stage to stage. Far greater knowledge than 

 mine would be required to compile such an itinerary. 



Mechanism. 



The first step in advance of Hofmeister's views was the esta- 

 blishment that the curvatures under consideration are due to 

 unequal growth— that is to say, to an excess of longitudinal 

 growth on the convex than the concave side. It is not, how- 

 ever, easy to say how far Hofmeister had this idea, for it, in 

 fact, depends on how we define "growth." Hofmeister knew, 

 of course, that the convex side of a curved shoot was longer 

 than it had been before the curvature occurred ; this is a 

 mathematical necesity. But he also made out the important 

 point that the concave side increases in length during the curva- 

 ture. These permanent elongations he must have known to be 

 growth, but his attention was directed to what is, after all, the 

 more important point— namely, 'Mhy'w. was that unequal elonga- 

 tion took place, 



Sachs, in his "Experimental-Physiologic," held that growth- 

 curvatures areduetounequal growth. Inhis "Text-book" (1874), 

 English translation, 1882, p. 853, the author, refening to Hof- 

 meister's work, says: — "I pointed out that the growth of the 

 under surface of an organ capable of curving upwards was 

 accelerated, and that of the upper surface retarded ; I did not 

 at the time express an opinion as to whether these modifications 

 of growth were due to an altered distribution of plastic material 

 or to a change in the extensibility of the passive layers of tissue." 

 Frank's already-quoted paper made valuable contributions to 

 the subject. He showed that the epidermic cells on the convex 

 side of the root are longer than those on the concave side— that 

 is, they have grown more ; he explained apogeotropic curvatures 

 in precisely the same way. He showed, moreover, that the 



