158 PLANT PHYSIOLOGY 



The significance of the profile position in bright light is on the whole 

 intelligible, for we have here to deal with a directive movement of individual 

 chloroplasts comparable with that we found in a motile leaf ; it enables the 

 chloroplast to obtain just the amount of light it requires. 



In addition to profile and surface positions the chloroplasts may, in many, 

 but by no means in all plants, take up a third, which has been termed the 

 night position (Fig. 172, N). The chloroplasts retreat from the outer walls, and 

 cover the inner and lateral ones, i. e. some are in the profile, some in the surface 

 condition. Obviously chemotactic movements co-operate here, for carbon- 

 dioxide more especially must have an attractive effect on the chloroplasts. 

 When assimilation ceases on darkening the carbon-dioxide must, according 

 to SENN (1904), accumulate on the inner and lateral walls, diffusing rapidly 

 through the outer walls. Owing to the influence of light, chemotactic move- 

 ments may be completely masked. 



Other movements of the chloroplasts, such as their aggregating round 

 the nucleus and in the corners of the cells, we need not discuss, for they are 

 as yet but little understood (comp. KUSTER, 1905). We need only note that, 

 in addition to regional changes, changes in form also occur. Thus chloro- 

 plasts on the narrow sides of palisade cells have the power under light of 

 bright intensity of changing from a hemispherical to a lenticular shape (SxAHL, 

 1880). They thus offer a reduced surface to the light, and hence their change 

 in shape has the same biological significance as the profile position in other 

 cases. 



In conclusion a word or two may be said as to the nucleus. 



1. 50, for traumotaxis read traumatotaxis. 



554, 11. 3-8, for We mention . . . nature, read It is a question whether their 

 nuclei may not have been passively squeezed into the neighbouring cells by 

 pressure exerted during the preparation of the object, but according to FARMER 

 and DIGBY (1907) the nuclei of certain fern prothalli may wander actively into 

 neighbouring cells. 



