THE SIGNIFICANCE OF CHLOROPHYLL 19 



seeking the light, and in another direction have de- 

 veloped the bacterial habit, which, as we have seen, 

 plays an important part in the economy of Nature. 

 But the fact that there are some Bacteria which are 

 independent of organic food, and derive all their food- 

 supplies from mineral sources, makes one think furiously : 

 these forms possess no chlorophyll, which has always 

 been regarded as essential in the transmutation of the 

 inorganic into the organic. Perhaps the purple pig- 

 ments peculiar to these exceptional Bacteria takes the 

 place of chlorophyll, but the existence of such forms 

 seems to point to the possibility of the existence of early 

 life-forms, which could build up organic compounds 

 without the assistance of chlorophyll. As Dr. Douglas H. 

 Campbell says in his Evolution of Plants : " The restric- 

 tion of this power to green cells is possibly a secondary 

 condition." 



We have several times alluded to the pigment chloro- 

 phyll, or leaf -green, and, as we are approaching the study 

 of green plants, it is essential that we should understand 

 the use and significance of this substance. It is well 

 known that a potato contains much starch; this consti- 

 tutes a reserve store intended for the nourishment of 

 young potato-plants. A potato will " sprout " with- 

 out being placed in the soil, and the sprouts will grow for 

 a time; growth, indeed, continues until the reserve 

 material in the tuber is exhausted. If the sprouting 

 takes place in the dark, the sprouts will not be green, 

 but they develop a green colour in sunlight. Even a 

 potato tuber lying on the soil will become green on the 

 surface exposed to the light, but the surface resting 

 on the soil develops no green pigment. In this and a 



