September 28, 1900.] 



SCIENCE. 



473 



of the wood-parenchyma are supposed to 

 absorb liquid froiQ the vascular tissue at 

 one level and force it back again by a vital 

 act at a higher level : this theory was dis- 

 posed of by the fact that the transpiration- 

 current can be maintained through a con- 

 siderable length of a stem killed by heat or 

 by poison. In the other, suggested by 

 DLson and Joly (1895-99), and also by 

 Askenasy (1895-96), it is assumed that 

 there are, in the trunk of a transpiring 

 tree, continuous 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 lbs. 

 to the square inch. But there is no evi- 

 dence to prove that a tension of anything 

 like twenty atmospheres exists, as a matter 

 of fact, in a transpiring tree ; on the con- 

 trary, such observations as exist (e. g., those 

 of Hales and 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, fortu- 

 nately, 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 fa- 

 miliar. The nineteenth century opened, 

 then, with a considerable store of facts ; 

 but what was lacking was an interpretation 

 of them ; and whilst it has largely added to 

 the store, its most important work has beea 

 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 irri- 

 table 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 bj^ 

 gravity, and at the same time Frank an- 

 nounced the important discovery that dor- 

 siventral members, such as leaves, behave 

 quite differently from radial members, such 

 as stems and roots, in that they are diageo- 

 tropic. 



It was a long time before the irritability 

 of plants to the action of light was recog- 

 nized. Chiefly on the authority of de Can- 

 doUe (to whom we owe the term 'heliotro- 

 pism ' ) , heliotropic curvature 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 co- 

 incides with the incident rays. Moreover, 

 the discovery by Knight (1812) of negative 

 heliotropism in the tendrils of Vitis and 

 Ampelopsis really put the Candollean 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 dia- 

 heliotropism of dorsiventral members. 



The question of the localization of irrita- 

 bility has received a good deal of attention. 

 The fact that the under surface of the pul- 



