272 



PHOTOSYNTHETIC SYSTEM. 



contrary, Stahl's experiments have conclusively shown that in diffuse 

 light (i.e. in relatively feeble illumination) the broad face of the chloro- 

 plast is always placed at right angles to the direction of the incident 

 rays (surface or full-face position), whereas in direct sunlight (i.e. in 

 intense illumination) the narrow edge is presented to the light (profile 

 position). It is quite an easy matter to observe the rotation of these 

 chloroplasts in response to changes of illumination. The advantage of 

 the mobility is obvious ; under favourable conditions of illumination 

 the chloroplasts expose their full surface, and by so doing are enabled 



to absorb the maximum amount of 

 light. In very bright light, which 

 is injurious because of the rapid 

 decomposition of chlorophyll in- 

 duced thereby, and also perhaps 

 for other reasons, they take up 

 the profile position and thus escape 

 the greater proportion of the light 

 that falls upon the cell. 



Chloroplasts which adhere 

 closely to the ectoplast obtain 

 the suitable intensity of illumina- 

 tion by moving along the cell-wall. 

 This arrangement is exemplified 

 by the tubular thallus of Vaucheri/t, 

 by the protenemal filaments of Mosses and by the basal filamentous 

 region of a Fern-prothallus. A very similar state of things prevails 

 in the leaves of Mosses, in the expanded portion of a Fern-prothallus, in 

 the fronds of Lcmna (Fig. 109), and in other organs composed entirely of 

 one or more layers of green parenchyma. In all these cases it is always 

 the difference in the intensity of illumination at different points of the 

 cell that controls the movements of the chloroplasts. The direction of 

 the incident rays is of no importance, except in so far as it is responsible 

 for the aforesaid difference of intensity. Senn's detailed investigations 

 have shown that chloroplasts in general contrive to take up their 

 position in those parts of the cell-wall which are for the time being 

 exposed to the most favourable intensity of illumination ; in moderately 

 intense diffuse light they move towards the illuminated tracts, while in 

 direct sunlight they seek the shaded portions. Thus, in the case 

 of a Moss-leaf or Fern-prothallus, the periclinal walls are covered with 

 chloroplasts in diffuse light, while the anticlinal walls are bare ; to this 

 condition Frank has given the name epistrophe. In the brighter 

 illumination produced by partially intercepted sunlight, the chlorophyll- 

 corpuscles shift from the tangential to the radial walls ; this arrange- 



Fig. 10S. 



Parenchymatous cell from the peripheral tissue 

 of a Potato-tuber, which has turned green through 

 exposure to light. The nucleus is connected with 

 the chloroplasts by protoplasmic strands. 



