6l8 LECTURE XXXV. 



them more in detail, and in doing so I shall make use of Stahl's very thorough work '. 

 He studied particularly the filamentous Alga Mesocarpus, in which the phenomena 

 in question are particularly clearly shown. ' The elongated cylindrical cells of this 

 Alga are connected into long filaments, and contain an axial band of chlorophyll 

 running the whole length of the cell, and the margins of which now and then 

 extend all round to the protoplasm lining the cell-wall ; in this case the whole 

 cell is divided by it into two approximately equal halves. The band of chlorophyll 

 is usually extended in one plane : on observing it from the surface the whole cell 

 thus appears uniformly green ; if the Alga is turned round through 90° so that 

 the band of chlorophyll is seen no longer from the surface, but in profile, the 

 otherwise transparent cell is traversed along its whole length by a dark-green 

 thin longitudinal strip.' If various filaments of this Alga are placed beneath the 

 microscope the observer sees these plates of chlorophyll from the surface, or in 

 profile, or in positions midway between these. ' Left to itself and undisturbed, 

 such a preparation presents after some time a different aspect — for it is found 

 that in the straight filaments all the bands are arranged in one plane. The 

 orientation of the plates is the same in all those which lie parallel to one another, 

 but diflfers in those which cross one another.' For the sake of simplicity we may 

 here take into account only those filaments which lie horizontally, and receive 

 the daylight (from a window) at right angles to their long axes. In this case it 

 is easy to demonstrate that the plates of chlorophyll all turn their broad surfaces 

 to the light, so that they receive its rays perpendicularly. If the direction of the 

 incident rays is altered, the plates of chlorophyll slowly turn, so that they always 

 present their surfaces at right angles to the rays of light ; in warm weather actively 

 vegetating plants complete these movements in a few minutes. Direct sunlight, 

 on the contrary, effects after a short time an entirely different disposition of the 

 plates of chlorophyll : they turn one edge towards the sun, or in other words 

 place their flat surfaces parallel to the incident rays. ' Light thus exerts a directive 

 influence on the chlorophyll apparatus of Mesocarpus. In feebler light it is disposed 

 perpendicularly to the path of the light (this is termed by Stahl the plane position), 

 in more intense light its plane coincides with the direction of the rays (he terms 

 this the profile position).' 



In the vesicles of Vaucheria (p. 108) there are rounded chlorophyll-corpuscles 

 embedded in the protoplasmic lining to the wall. According to the intensity of 

 the light these also are arranged in a manner which is understood most simply by 

 regarding the chlorophyll-corpuscles themselves as parts of the plate of chlorophyll 

 described in Mesocarpus. If the intense light persists for some time, the chlorophyll- 

 corpuscles become collected into distinct heaps — a process which De Bary saw 

 occurring with great energy in the vesicles of Acetabular ia (a marine Alga), and de- 

 scribed as follows : — ' If actively growing vesicles a few mm. long receive the rays of 

 the sun directly, the protoplasm which carries the chlorophyll instantly rounds off 

 and contracts into irregular clumps. The individual grains are seen to leave their 

 place rapidly, and as it were tumble up against one another, becoming collected 



' Stahl, 'Ober den Einjluss von RkhHmg und Starke der Beleuchtting,' in Bot. Zeit., 1886 

 (P 297), where the literature of this subject is also given in detail. 



