162 THE CELL 



Several small starch granules are generally enclosed in the 

 chlorophyll corpuscles, being formed in them through assimilation. 

 They are most easily seen, if, when the chlorophyll has been ex- 

 tracted by means of alcohol, tincture of iodine is added to the 

 preparation. 



As has been proved by Stahl's investigations, the chlorophyll 

 granules, quite apart from the changes of position brought about 

 by the streaming movements of the protoplasm (vide p. 104), are 

 able to change their shape under the stimulating influence of the 

 sun's rays, to a surprising extent. Whilst in diffused daylight they 

 assume the shape of polygonal discs with their broad sides directed 

 towards the source of light, in direct sunlight they contract up 

 into little round balls or ellipsoidal bodies. By this means they 

 effect a change which is necessary for the performance of the chloro- 

 phyll function, by " offering to direct sunlight a small surface, and 

 to diffused daylight a larger one, for the absorption of the rays of 

 light. In this, they offer us an insight into the high degree of 

 the differentiation that they have attained which we could never 

 have arrived at simply by the study of their chemical activity ' 

 (de Vries V. 46). As regards their mode of multiplication by 

 division, their active motility, their functions in the processes of 

 assimilation, etc., they appear, like nuclei, to be very highly 

 specialised plasmic products. 



Finally another variety of trophoplasts, the colour-granules, 

 must be mentioned : the red and orange red coloration of many 

 flowers is caused by their presence. They consist of a proto- 

 plasmic substratum which may assume very various forms, oc- 

 curring sometimes in the shape of a spindle and sometimes of a 

 sickle, a triangle or a trapezium. In this substratum crystals of 

 colouring matter are deposited. In this case also colourless tro- 

 phoplasts may, in suitable objects, be seen to develop gradually 

 into colour granules. Further Weiss has observed spontaneous 

 movements and ch angles of form in these granules also. 



We will conclude this review of the various kinds of tropho- 

 plasts by describing in more detail the structui'e of the starch 

 grains, which have acquired considerable theoretical importance 

 in consequence of Nageli's (V. 17, 20) researches, and the con- 

 clusions which have been deduced from them. 



The starch grains (Fig. 69) in a plant cell may vary consider- 

 ably as to size. Sometimes they are so small that even with the 

 strongest powers of the microscope they only appear as minute 



