268 PHOTOSYNTHETIC SYSTEM. 



change colour if they are exposed for long periods to coloured light, 

 the alteration always consisting in a closer approximation to the tint 

 which is complementary to that of the light employed. In red light, 

 for example, the plants take on a green, in green light a red, and in 

 blue light a brownish yellow hue. 



Stahl has even extended the views based on these interesting 

 discoveries to the Higher Plants, by suggesting that the chloroplast 

 pigments of the latter have been evolved in adaptation to the mixed 

 composition of ordinary white light. The green fraction of the crude 

 chlorophyll extract comprising chlorophyll proper serves, in Stahl's 

 opinion, for the absorption of the orange and red rays which pre- 

 ponderate in sunlight that has passed through the cloudy atmosphere 

 of the earth. The yellow portion, on the other hand, composed of 

 the xanthophylls, is concerned with the absorption of the blue and violet 

 rays which form a great part of the diffuse light reflected from the 

 blue sky. 



Chloroplasts are in many cases provided with characteristic 

 inclusions ; among the most widely distributed and important of these 

 are the familiar included starch-grains. 140 These bodies usually belong 

 to the class of compound grains. While the more or less numerous 

 constituent granules are quite young, they are separated from one 

 another by green protoplasmic material ; later, when the grains have 

 reached a considerable size, the actual substance of the chloroplast may 

 be reduced to a thin film surrounding the whole inclusion, or may 

 apparently even be ruptured by the latter at certain points. These 

 included starch-grains do not all arise in the same way. The material 

 utilised for their production may be photosynthetically manufactured 

 by the enveloping chloroplast itself; on the other hand, the latter may 

 merely convert into starch carbohydrate material that has been 

 imported from without in a soluble form. Hence the mere presence 

 of included starch-grains cannot be accepted as conclusive proof of 

 antecedent photosynthetic activity. Conversely even the most active 

 photosynthesis may not lead to the accumulation of included starch, if 

 the translocation of synthetic products is sufficiently rapid. The latter 

 condition is frequently fulfilled in the case of palisade-cells and other 

 specialised photosynthetic elements ; on the other hand, chloroplasts in 

 spongy-parenchyma cells or in the cortex of stems, although exhibiting 

 relatively slight photosynthetic activity, nevertheless retain their in- 

 cluded starch-grains for long periods, when these have once been formed. 



Chloroplasts almost always also contain inclusions of the nature of 

 oil-drops ; these vary in size, and are soluble in alcohol. As a rule, 

 they appear only in the older cells; in Vaucheria, however, oil-drops 

 are found adhering even to the young chloroplasts, while in the genus 



