THE SIMPLEST FORMS OF LIFE 37 
being exceedingly resistant to extremes of heat and 
cold that wonld 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 Cyanophycee has 
been given to this class, because in addition to the 
chlorophyll they usually possess a blue pigment (phyco- 
eyanin) which is readily soluble in water. 
The similarity in the structure and reproduction of 
the Cyanophycee 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. 
