lOZ THE PLANT CELL. 



along the upper and under walls, presenting their broader 

 surfaces to the light (epistrophe). This is an instance of adapta- 

 tion so as to ensure an " optimum " intensity of assimilation.* 

 'In Mougeotia (one of the Conjugatae) the chloroplast is in the 

 form of a band in each cell of the filament, and this band rotates 

 into a position which enables it to receive the optimum intensity 

 of illumination; in strong illumination it presents its edge to 

 the incident rays. 



h. The Influence upon Metabolism of Light Rays of Vary- 

 ing Befrangibility. — An experiment devised by Engelmann 

 (described in the Botan'n<die Zeitung for 1881, p. 447) illustrates 

 in a striking manner the influence of the red, yellow, and violet 

 portions of the spectrum upon the intensity of assimilation of 

 COj. A filament of Spirogyra is mounted in water along the 

 middle of an opaque microscope slide, so as to traverse three 

 transparent circular portions of the slide (see a. Fig. 115). These 

 three spaces are illuminated by red, yellow, and violet light, 

 R, Y, and V respectively (see Fig. 115). A small culture of 

 Proteus vulgaris (a motile organism which is markedly attracted 

 by oxygen) is now introduced under the cover-slip of the 

 preparation, and, the filament being carefully focussed under 

 the microscope, the following obser\ations may be made : — 



i. In the vicinity of those cells of the filament illuminated by the red 

 Fays, a vast swarm of motile bacteria become aggregated. 



ii. Fewer bacteria exist in the region of the cells illuminated by the 



yellow rays. 



iii. Very few organisms are to be seen near the cells lighted by 

 the violet portion of the spectrum. Engelmann employed a substage 

 prism in order to produce a spectrum, the red, yellow, and violet portion 

 of which thus illuminated the spaces in the slide from beneath. 



This shows that more oxygen is being evolved from those 

 cells of the filament illuminated by the red ra3s than from those 

 under the influence of the yellow or violet rays, and that, therefore, 

 assimilation is more intense in red than in other illumination. 



Another experiment, due to TimiriazefF, and depending upon 

 the formation of starch in the leaf-cells, is as follows (see Fi" 

 115):— 



A given leaf of a plant is selected on a certain daj-, and, before 

 any light has fallen upon it, is covered, with the exception of a 



* The small leaves of Lemna trisulca are excellent organs for the 

 observation of this phenomenon. 



