LOCOMOTORY DIRECTIVE MOVEMENTS 



553 



the external walls are always free of chloroplasts. This night position is not 

 found in all plants ; in many shade plants and aquatics all the chloroplasts are 

 superficially arranged in darkness. In plants which distinctly prefer bright 

 sunlight, the nocturnal profile position may be assumed on subjecting them to 

 light of relatively slightly diminished intensity. The day profile position also 

 occurs in various plants when subjected to light of varying intensity, an in- 

 tensity which is feeble in shade-loving plants and high in those which prefer 

 bright sunlight. Even in the cells of the same leaf differences manifest them- 

 selves, for the cells of the under-side of Elodea arrange their chloroplasts in the 

 superficial position before those of the upper side (MooRE, 1887). 



The significance of the profile position in bright light is generally intelligible, 

 for this position enables the chloroplasts to ar- 

 range themselves in such a manner that they 

 may receive the exact amount of light they re- 

 quire, just as a leaf can by changing its position. 

 The nocturnal profile position has, however, yet 

 to be explained. [It is obvious that in produc- 

 ing the nocturnal position chemotactic move- 

 ments co-operate, and carbon-dioxide more es- 

 pecially must play a part in determining the 

 position of the chloroplasts, for it must accumu- 

 late on the inner and lateral walls of the cells and 

 be less apparent on the outer walls (SENN, 1904).] 

 How the two positions are arrived at in Meso- 

 carpus has already been noted ; we have no 

 information, however, as to whether the move- 

 ments are active or passive, although if they be 

 passive they are more readily intelligible in the 

 present instance. 



It is impossible for us to enter more fully 

 into the discussion of certain other phenomena 

 presented by chloroplasts closely related to those 

 we have discussed, such as the aggregation of 

 the chloroplasts in the angles of the cells under 

 high intensities of light and their change of form 

 in palisade cells ; we must content ourselves 

 with observing that the external brighter or 

 darker green colour of the plant is often due to 

 changes of position of the chloroplasts. 



Passive changes in position of the nucleus 

 may generally be observed whenever the proto- 

 plasm shows signs of vigorous movements, and 

 these movements are autonomous in their 

 nature. Induced movements and movements 

 towards a definite region take place after wound- 

 ing. TANGL (1884) was the first to show that injury to the epidermis of the 

 scales of the onion induced a movement of the nuclei towards the surface 

 which had been wounded ; in that region also the protoplasm tended to collect. 

 To NESTLER (1898) belongs the credit of having demonstrated the very general 

 occurrence of traumotaxis, but it is to NEMEC (1901) that we owe the most 

 thorough investigations on the subject, and especially the determination of the 

 rapidity with which the stimulus due to injury travels and the induced position 

 is replaced by the normal. For all the new facts on this subje t we must refer 

 our readers to NEMEC'S papers. MIEHE (1901) and KORNICKE (1901) have also 

 recorded remarkable observations as to the migration of nuclei. Under certain 



Fig. 172. Transverse sections through 

 the frond of Lemna trifulca. ZJ position 

 of chloroplasts in diffuse light; 5", in 

 bright light ; N, at night. After STAHL, 

 1880. From the Bonn Textbook. 



