THE SIMPLEST FORMS OF LIFE 37 



being exceedingly resistant to extremes of heat and 

 cold that would be fatal to most plants of higher 

 organization. They often occur in thermal springs 

 which are impregnated with various substances usually 

 inimical to plant life. The name Cyanophycese has 

 been given to this class, because in addition to the 

 chlorophyll they usually possess a blue pigment (phyco- 

 cyanin) which is readily soluble in water. 



The similarity in the structure and reproduction of 

 the Cyanophyceae and Bacteria have led botanists to 

 unite them into a common group, the Schizophyta, 

 based upon the prevailing method of reproduction by 

 simple, transverse fission. Whether this group is di- 

 rectly related to any other group of plants is question- 

 able ; but there is good reason to suppose that they 

 represent an extremely primitive type of vegetation, 

 and it has even been suggested that similar organisms 

 were probably among the first to make their appearance 

 upon the earth before the conditions were fit for higher 

 forms of plant life. 



It seems probable that the earliest forms of life could 

 manufacture carbon-bearing compounds without pos- 

 sessing chlorophyll, and that the restriction of this 

 power to green cells is a secondary condition. 1 



While both the Slime-moulds and Schizophytes show 

 but doubtful affinity with the higher plants, there is a 

 third group of low organisms, sometimes united with 

 these two under the name of Protophytes, which are 

 of especial interest in connection with the evolution of 



1 Certain bacteria, although destitute of chlorophyll, are independent 

 of organic food. Such forms, however, possess a red or purple pig- 

 ment, which serves as a substitute for chlorophyll. 



