MEASUREMENT OF LIGHT 127 



from such a calamity by moving the chloroplasts out of the direct 

 light. That is, if a plant remains in direct sunlight for a time, its 

 chloroplasts are usually found to be arranged mostly along the walls 

 that are perpendicular to the leaf surface, where they are somewhat 

 protected from the light rays. On the other hand, if the same plant 

 is exposed to dim light, its chloroplasts gradually assume positions 

 mostly along the walls that are parallel to the surface, where they 

 receive as much light as possible. One of the filamentous algae, 

 Mongeotia, shows somewhat comparable chloroplast movements. 

 In this alga there is a single large chloroplast in the shape of a 

 rectangular plate in each cell. When this alga is in diffuse light the 

 chloroplasts present their broad faces to the incident rays of light 

 but when in bright light they turn so that the edge of the plate is 

 toward the light. 



Certain plants, especially Lactuca scariola, the prickly lettuce, and 

 Silphium laciniatmn, the rosin weed, are called compass plants be- 

 cause most of their leaves are edgewise and in a north and south 

 plane. This condition holds, however, only for plants growing in full 

 sunlight, the leaves of the same species growing in partial shade being 

 horizontal. It is fairly certain, therefore, that the vertical position 

 of the leaves of compass plants is a result of light conditions. It is 

 probable that this position protects the chloroplasts from too great 

 an intensity of light, since the sun's rays- are perpendicular to the 

 flat side of the leaves only in the morning and the evening when the 

 light is much less intense than during the middle of the day. 



A similar protection is had by such plants as Iris, Typha, and 

 many grasses, the leaves of which are vertical. The position of such 

 leaves, however, is probably not a response to light conditions 

 entirely since they are held in position by their sheathing bases, and 

 probabh' a greater advantage than that of protection from intense 

 light is that light is permitted to penetrate nearly or quite to the 

 bases of the leaves. This enables the entire length of the leaf to be 

 utilized for photosynthetic activity. 



78. Measurement of Light.— ^Measurements of light are usually 

 relative rather than absolute, that is, the result of such measure- 

 ments are expressed in units which represent, not the absolute 

 amount of light, but the amount as compared to some other amount 

 which is used as a standard. The method that has been used more 

 than any other is the tinting of silver chloride paper ("printing out" 

 paper) to a standard tint and noting the time required to reach that 

 tint. The chief objection to this method is that the silver chloride 



