CHLOROPHYLL AND LIGHT INTENSITY. 383 



to the naked eye as green stripes and patches. If swarm-cells of Sphcerella 

 pluvialis are cultivated in a flat white china dish filled with rain-water, and one- 

 half of the dish is darkened by means of an opaque body while the other half 

 remains illuminated, the whole of the swarm-cells swim from the darkened to the 

 illuminated water in order to take up a position as favourable as possible with 

 regard to the light. If now the china dish is turned round so that the hitherto 

 illumined portion becomes darkened and light falls on the part previously obscured, 

 the swarm-cells forsake the position which they had recently taken up, swim from 

 the now darkened place to the illuminated side opposite, and arrange themselves 

 there according as the illuminating conditions are favourable. 



If, instead of the Sphcerella pluvialis discussed above, clumps of Vaucheria 

 clavata are cultivated in a china dish filled with water, and the water is again 

 partially darkened, together with the green tufts growing in it, it will be seen that 

 the cells, which are elongated and fixed at one end, seek with the other end those 

 places where they can find the best light. Vaucheria clavata, which has been 

 repeatedly cited as an example, and which is represented in the middle figure on 

 page 139, consists of long tubular cells, frequently bulging and branched, whose 

 blunt growing ends appear dark green, while the lower dead portions are branched 

 and coloured yellowish-white. The protoplasm is so richly studded with chlorophyll- 

 granules that the entire inner wall of the tubular cells appears covered with a green 

 lining. At the bottom of shallow pools, which is the natural habitat of these 

 plants, they form hemispherical clumps, and all the tubular cells which compose 

 the clumps have their green ends directed upwards towards the source of light. 

 The same thing occurs when the Vaucheria cultivated in the china dish is 

 uniformly illumined from above; but, if partially obscured, those filaments over 

 which the darkening shadow is thrown very quickly alter their position. They 

 bend towards the light side, and then the clump looks just as if its filaments had 

 been combed in this direction. Moreover, the same thing is also seen when the 

 china dish containing clumps of Vaucheria (on which until now diffused light has 

 fallen uniformly from above) is placed at the further end of a one-windowed room, 

 so that the light can only reach it from one side. Here, again, all the filaments, or 

 rather, tubular cells of the clump, bend towards the source of light, and if they 

 continue to grow, the increase in length is universally in a line with the direction 

 of the incident rays. After a few days these Vaucheria clumps also look as if they 

 had been combed out. 



The green tissues of thallophytes, and the green leaves and stems of ferns, and 

 phanerogams, i.e. those extensive combinations of green cells whose function is to 

 work in a harmonious manner, and to manufacture organic substances for the 

 plant to which they belong from carbonic acid with the help of other food- 

 materials; these behave in the same way as the individual green cells which 

 swim freely in water, and as the tubular cells of Vaucheria, which are attached 

 at one end. Arrangements are necessary for these likewise, by which they can 

 always be placed in the most favourable light. Of course, in these plants where 



