PHYSIOLOGY 269 



are at right angles to the direction of the rays of light (Fig. 220 T). 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. 220 S). 

 In darkness or in weak light the chloroplasts group themselves in still a third way 

 (Fig. 220 A"), the advantage of which is not altogether clear. Similar changes of 

 position may result from certain substances in solution in the surrounding water. 



The form of the chlorophyll bodies themselves undergoes modification during 

 changes in their illumination ; in moderate light they become flattened, while in 

 light of greater intensity they are smaller and thicker. 



As a special mode of protection against too intense light, the chloroplasts of the 

 Siphoneae and Diatomeae (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 chloroplasts, the colouring of green organs naturally 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 movements of the 

 chloroplasts by the accidental observation that the shadow of a thermometer was 

 represented in dark green on a leaf otherwise directly illuminated by the sun. 



Wounds and cell-wall thickenings localised to one side of the cell likewise give 

 rise to orientation movements, as they occasion a crowding together on one side 

 of the nucleus and protoplasm. 



A peculiar mode of protoplasmic movement, which may be termed 

 STREAMING, occurs, according to ARTHUR, in the non-septate mycelium 

 of some Fungi. In it the whole mass of protoplasm, with its included 

 vacuoles, streams towards the end of the hypha, only the limiting 

 layer remaining at rest. After a longer or shorter interval a similar 

 streaming movement sets in in the opposite direction (cf. Fig. 221). 



TERNETZ observed the same type of movement in the mycelium of Ascophauus 

 carneus, where the protoplasmic stream, with its vacuoles, makes its way through 

 the pores of the perforated transverse septa. In this latter case it was established 

 that the direction of streaming was determined by local differences in the supply 

 of water C 77 ). 



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. 175). 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 

 opposite side becomes shorter or longer, and also from the unequal 

 elongation or contraction of both sides, or from the elongation of one 

 side and the contraction of the other. 



Such curvatures most frequently occur in plants as a consequence 

 of UNEQUAL GROWTH. More rarely they are due to the different 



