552 TRANSFORMATION OF ENERGY 



medium. We are led to believe, therefore, that such an organism must possess 

 the power of appreciating how deep it is in the medium ; such a sensitiveness 

 cannot arise in any way from the direct action of gravity but it may be due to 

 its power of appreciating the pressure of the liquid upon it. In fact Jensen 

 (1893) has endeavoured to refer geotaxis to perception of this pressure, although 

 he was unable to offer exact proof of the truth of his hypothesis. 



Tactic movements are not limited only to free organisms ; they occur also 

 in protoplasm enclosed within cell-walls, and especially in certain organs of the 

 cell, such as chloroplasts and nuclei. In the former case we are acquainted with 

 remarkable phototactic movements, in the latter the movements are trauma- 

 totactic arising especially after wounding. The movement of chloroplasts may 

 be best studied in Mesocarpus ; only one chloroplast, which takes the form of a 

 flat, rectangular band, is present in each of the cylindrical cells of this alga. 



In Fig. 171, /, the band is shown, in a transverse section of the cell, in the 



position which it takes up when subjected to light of medium intensity ; the 



chloroplast behaves itself under these conditions just like a heliotropic leaf 



placing itself at right angles to the incident ray, and hence, presenting its greatest 



surface to it. If the intensity of the light increases, however, the chlorophyll 



plate twists round through an angle of 90° and finally presents its edge to the 



light (correspondnig to the profile position of the foliage-leaf). Experiments in 



illustration of this fact may be readily carried out ; 



y . they have been favourite lecture experiments ever 



^ y^ since the phenomenon was first described by Stahl 



j^ N- y' V (1880). Nevertheless there are many important 



/^k \ f J^\ details with regard to these movements which have 



V ^^y \w ) ^ot y^^ been explained, for instance, is the chloro- 



^ ^ V ^ phyll itself active ? in answer to which it must be 



/ ^ admitted that motile organs in connexion with the 



Fig. J71. Diagrammatic trans- chloroplast havc not yct bccu discovcrcd. Does 

 ^«o<ra*%!f°"The'ch?oropiasTis re- it movc passivcly ? How comcs it about then that 

 Sre^'^the ^a'rrorts^lndtcati" [he ^^^ activc protoplasm twists the chlorophyll plate 

 direction of the light. Only Until it rcachcs the desired position ? Were 



it simply the case that the protoplasm lying on 

 the side previously receiving the most light moved away from that side as the 

 intensity of the light increased and passively carried the chloroplast with it, 

 then the whole phenomenon would be perfectly intelligible. But as a matter 

 of fact the protoplasma touching one edge of the chloroplast must perform 

 a movement exactly opposite to that touching the other edge. It is quite 

 inexplicable how it is that after sufficiently long illumination the twisting of 

 the chloroplast is carried out in the dark only to the same extent as it would 

 be if continuously illuminated (Lewis, 1898). How does the plate know when 

 it has turned through an angle of 90° ? 



The phenomena concerned in the movements of many small chloroplasts in 

 a cell are more easily understood. Here also we meet with a surface and a profile 

 orientation but these arise not by simply twisting the chloroplast in situ but 

 by transferring them to more appropriate situations. The surface orientation 

 is attained when the chloroplasts arrange themselves over the illuminated wall 

 of the cell, the profile position when they lie parallel with the path of the incident 

 ray, and therefore at right angles to the surface of the cell exposed to light. 

 Fig. 172 represents a transverse section of the frond of Lemna trisulca, the arrows 

 indicating the path of incidence of the light rays. At T the chloroplasts are 

 represented in the superficial position which they assume in diffuse light, at S 

 they have taken up the profih position in direct sunlight. It must be noted, 

 however, that a third, or night orientation may be observed in which some of 

 the chloroplasts are in the profile, some in the superficial position, but where 



