BOTANY 



light (Fig. 199, T). 



Fig. 199. — Varying positions taken by 

 the chlorophyll grains in the cells of 

 Lemna trisulca in illumination of dif- 

 ferent intensity. T, in diffuse day- 

 light ; S, in direct sunlight ; N, at 

 night. The arrows indicate the direc- 

 tion of the light. (After Stabl.) 



They quickly pass over to the walls parallel to 

 the rays of light, however, as soon 

 as the light becomes too intense, 

 and so retreat as far as possible from 

 its action (Fig. 199, S). In darkness 

 or in weak light the chloroplasts group 

 themselves in still a third way (Fig. 

 199, N), the advantage of which is not 

 altogether clear. 



The form of the chlorophyll bodies 

 themselves undergoes modification dur- 

 ing changes in their illumination ; in 

 moderate light they become flattened, 

 while in light of greater intensity they 

 are rounded and thicker. 



As a special mode of protection against 

 too intense light, the chloroplasts of the 

 Siphoneae (and the same thing is observed 

 in many plants) become balled together in 

 separate clumps. In correspondence with 

 the changes in the position of the chloro- 

 plasts, the colouring of green organs natur- 

 ally becomes modified. In direct sunshine 

 they appear lighter, in diffused light a 

 darker green. The attention of Sachs was 

 first called to the phenomena of the move- 

 ments of the chloroplasts, by the accidental 

 observation that the shadow of a thermo- 

 meter was represented in dark green on a 

 leaf otherwise directly illuminated by the 

 sun. 



Wounds and one-sided cell-wall thicken- 

 ings likewise give rise to orientation move- 

 ments, as they occasion a crowding together 

 on one side of the nucleus and protoplasm. 



Movements producing Curvature 



The movements of the organs of stationary plants, unicellular as 

 well as multicellular, are accomplished by means of curvatures. In an 

 organ that has grown in a straight line the longitudinal sides are all of 

 equal length ; in an organ that is curved, however, the concave side is 

 necessarily shorter than the convex side. When, accordingly, the 

 opposite sides of a pliable organ become of unequal length, the organ 

 must curve toward the shorter side (Fig. 168). Inequality in the 

 length of the opposite sides may result from various causes. A 

 curvature occurs if the length of one side remains constant, while the 



