38 PLANT LIFE. 



minute particles called chlorophyll grains. A chlorophyll 

 grain consists of two parts : a solid, colourless portion 

 consisting of modified protoplasm, having a spongy texture 

 called the chromoplast, and the green colouring matter or 

 chlorophyll which fills the cavities of the chromoplast. 

 The latter becomes differentiated while the cell is quite 

 young and in total darkness, and in this condition becomes 

 coloured yellow. At this stage development is retarded 

 until the cell is exposed to light, which is necessary for the 

 production of the green chlorophyll, as is also the presence 

 of iron in solution, although this substance does not enter 

 into the composition of chlorophyll. Its presence is probably 

 necessary for the purpose of aiding in the chemical changes 

 that result in the formation of chlorophyll. From what has 

 been said as to the necessity of light for the formation of 

 chlorophyll, it will be understood why this substance is only 

 present in the superficial parts of plants exposed to light ; 

 and further, its most important function can only be 

 exercised in the light. Nevertheless chromoplasts exist 

 in structures that are normally developed in darkness, and 

 on being exposed to light become green, as is often seen in 

 potatoes that become exposed during growth. Exceptions 

 to the above statement respecting the importance of light in 

 connection with the development of chlorophyll are not 

 unknown : the prothallus in ferns and the cotyledons or 

 seed-leaves of Conifers become green in total darkness. 

 The significance of this exception is not understood. In 

 some plants, as red and brown seaweeds, the purple beech, 

 etc., the green colour of the chlorophyll is masked by the 

 presence of an additional colouring matter present in the 

 cells. 



The important work done by chlorophyll consists in the 



