380 CHLOROPHYLL AND LIGHT INTENSITY. 



amount of chlorophyll in what manner the sun's rays light upon them, or what 

 quantity of vital force is transmitted to them in a given time. Various species of 

 plants may make very dififerent demands for sunlight, but for each individual 

 species the need of propelling force fluctuates only within very narrow limits, which 

 cannot be exceeded without injury. The greatest possible equality in the supply of 

 propelling force is an indispensable condition of a successful career. In order to 

 meet the inequality in the flow of light on bright and dull days, and also during 

 various parts of the day, it is arranged that the green organs can turn towards the 

 sun, and that according to the hour of the day and the strength of the sun's rays 

 at that particular time, they can take up a definite position, and again alter this 

 position with ease. And, indeed, the green chlorophyll-granules in the interior of 

 the cells also show this capability of accommodating themselves in accordance with 

 the demand for light as well as the entire cells, and, finally, even the green leaves, 

 together with the stems and branches which bear them. 



If one would obtain a clear idea of the withdrawal of the chlorophyll-granules 

 from the sunlight, one must remember, first of all, that these green bodies, what- 

 ever may be their form, are imbedded in the protoplasm of the cell, and that the 

 protoplasm is mobile and easily capable of displacement — or, in other words, that 

 the protoplasm which contains the green chlorophyll-granules twists and rotates 

 within the cell it inhabits, and can transport the granules hither and thither. Still 

 more. Chlorophyll-granules can be temporai'ily heaped up and crowded together in 

 definite places ; they may again be separated from one another, and distributed 

 equally throughout the whole cell-body. In the tubular cells of Vaucheria 

 clavata, represented in figure 25a, a, the protoplasm forms a lining layer on 

 the inner side of the colourless transparent cell-wall, and is so thickly studded 

 with round chlorophyll -granules that the cell appears of a uniform dark green. 

 But this is only the case with light of moderate intensity. When strongly illumi- 

 nated the chlorophyll-granules move apart from one another, arrange themselves in 

 isolated balls, and in a very short time, in each tubular cell, dark-green spots and 

 zones may be seen corresponding to the crowded granules, and light, irregular 

 bands appearing in those places from which the chlorophyll has been withdrawn. 

 If the intensity of the light diminishes, the green clusters dissolve, and the former 

 equal distribution and colouring is resumed. In another filamentous green alga, 

 which lives in water and belongs to the genus MesocarjDUs, each of the long 

 cylindrical cells contains a plate-like chlorophyll body, which in weak diffuse light 

 turns itself at right angles to the incident rays. In this position the broad side, i.e. 

 the larger surface of the chlorophyll body, is turned to the sun, and the incident 

 light is in this way utilized to the utmost possible extent. As the plate-like 

 chlorophyll body usually extends right across the cell, under the conditions indicated, 

 the cell appears of a uniform green colour. If the full rays of the sun fall on such 

 Mesocarpus cells, the plate-like chlorophyll bodies begin to turn so that the plane 

 of the plate is parallel to the direction of the rays. Now the narrow side, i.e. the 

 smaller surface of the chlorophyll body, is turned to the rays, and only a dark-green 



