Evolution in the Plant Kingdom. 327 
In this lowest chamber of Protophytes, every now and then the 
garment of sexual reproduction is discovered, and its wearer invited 
to take a place in some upper chamber. But the chances are that 
the chamber will never be completely emptied, and that there will 
always be some plants called Protophytes. 
In the second group we would expect to find the beginning of 
sex-reproduction in its simplest form; and to understand what the 
simplest form would be, the nature of sex-reproduction must be 
defined. It consists in the mingling of the contents of two cells 
to form a new one. This new cell is the progeny, and develops 
more or less directly into the structure of the parents. 
Applying this definition to some one-celled form as Protococcus, 
the simplest possible method of sex-reproduction would be for two 
cells to come in contact and mutually discharge their contents into 
a blended mass which becomes a new cell and presently resembles 
the parents. Such is the beginning of sex-reproduction as we find 
it in the second group of plants; but it will be noticed that there 
is no distinction of sex. Both cells are constructed alike and act 
alike ; neither is receptive, for the new cell is constructed upon neu- 
tral ground. Sexuality has been attained, but not bisexuality. 
For this reason, the second plant group is frequently called -the 
“Unisexual Group”; or, from the fact that the cells are for a time 
yoked together, they are technically called Zygophytes, or “ Yoked 
Plants.” In this group, not only is sexuality begun, but bisexu- 
ality is hinted at. Plant bodies now begin to consist, not of single 
cells, but of cell-groups, usually arranged in a chain, forming fili- 
form or thread-like plant bodies. In these filaments or chains of 
cells, any cell (for they are all alike) can become a reproductive 
cell and join issues with any other cell, either in the same filament . 
rin another. There is no setting apart of special cells to do this 
special work, for it is done equally well by all, and all are ordinary 
vegetative cells. The first hint at bisexuality comes with the fact, — 
that one of these conjugating cells becomes receptive, receiving the 
contents of the other, and within it the spore or progeny cell is 
formed. 
Such is the case in the common Spirogyras, or “ frog-spittles.” 
Although one cell becomes receptive, there is no difference in form 
nor in contents, and it seems immaterial which becomes the receptive 
one. In other forms, the development of the spore within acai 
