CAUSES OF THE MOVEMENTS OF THE CHLOROPHYLL-CORPUSCLES. 62I 



the opinion, based on Frank's observations, that the chlorophyll-corpuscles are passive 

 in the matter, and that the movements referred to belong to the protoplasm itself 

 in which they are embedded. Frank and Stahl agree with this view : if it is right, 

 however, as is hardly to be doubted, then all the statements so far made as 

 to the chlorophyll-corpuscles apply properly to the protoplasm itself, thus confirming 

 and more exactly characterising its sensitiveness to light. 



Again, the change in form of chlorophyll-corpuscles in varying illumination, 

 already demonstrated by Micheli in a piece of work done with me in 1866, was 

 confirmed and more exactly described by Stahl. In the Moss Fimaria he found 

 that in diflfuse daylight the chlorophyll-corpuscles are disposed on the outer surfaces 

 of the cells neariy touching one another, and separated only by narrow strips of 

 colourless protoplasm. The grains are in this condition flat and polygonal. On 

 exposure to direct sunlight they withdraw their projecting angles and become 

 rounded and smaller in circumference, and thus further apart. Similar phenomena 

 were also observed by Stahl in the so-called palisade parenchyma of the leaves of 

 Phanerogams, and it is only necessary to add here that in sunshine, as in shade, 

 the chlorophyll-corpuscles of these cells have the so-called profile position — i. e. are 

 situated on the walls parallel to the rays of light ; in the shade the chlorophyll- 

 corpuscles are in this case approximately hemispherical, in the sun more flattened 

 and discoid. Stahl demonstrated similar changes also in the assimilatory paren- 

 chyma of those Liverworts which have flat extended shoots, and was able to detect 

 the phenomenon generally wherever he sought for it. At the same time it should 

 be remarked that in some leaves, clump-like aggregations of the chlorophyll-corpuscles 

 are formed in the palisade cells by continued insolation. 



From all his numerous observations Stahl concludes then as follows : ' In 

 feeble illumination the largest surface of the chlorophyll-corpuscle is turned towards 

 the source of light ; the light is absorbed as much as possible. An opposite 

 behaviour is noticed in very strong illumination : a smaller surface is presented 

 to the light. One and the same end is attained in utterly different ways ; the 

 chlorophyll-bodies sometimes protect themselves from too intense illumination 

 by turning round (^Mesocarpus\ and sometimes by travelling, or by alterations in 

 shape. These phenomena in the chlorophyll-apparatus proceed, again, hand in 

 hand with the position assumed by entire leaves in very strong or feeble illumin- 

 ation ; this also was investigated in detail by Stahl. As we shall see later on, 

 many leaves have the power of placing themselves in feeble light by means 

 of peculiar mechanisms, in such a position that the rays of light fall upon their 

 surfaces perpendicularly, whereas intense light induces them to assume the profile 

 position — i. e. to turn one edge to the sun, and thus place their surfaces parallel 

 with the sun's rays, thereby avoiding a too strong influence of the latter. Arrange- 

 ments of the most various kind combined with corresponding irritabilities for light 

 thus exist in plants in order to bring, not only the assimilatory apparatus, such as a 

 whole multicellular organ, but also its individual chlorophyll-bodies into particular 

 positions, which we must in any case regard as favourable for the employment 

 of the light. 



With reference to the heliotropic phenomena to be described later, and parti- 

 cularly with reference to my theory of heliotropism, I must notice two other important 



