328 Evolution in the Plant Kingdom. 
receptive cell demands more or less change of form, thus making a 
cell differing in appearance from the ordinary ones. To sum up 
the general phases of this advance in the second group, or Zygo- 
phytes; sexuality is attained, at first with no distinction of sex ; 
then one cell becomes receptive, but differs in no respect from any 
other in form or contents; and finally, the receptive cell becomes 
more or less changed in form by the development of the spore. 
In the third group we would expect to find bisexuality distinctly 
worked out, but of the simplest kind. The simplest kind of dis- 
tinct bisexuality would consist in setting apart two cells for the 
special performance of this function, differing from the ordinary 
cells of the plant body and from each other in form and contents. 
Naturally the receptive or female cell, in which the spore is to 
` develop, would be the larger, probably the largest cell produced by 
the plant. Such is the average condition of sexuality in the third 
group, called Odphytes, or “ egg-spore plants,” in reference to their 
large spores. It is to be noticed that these male and female cells 
differ in form and function only from the ordinary cells of the 
plant body ; they are not favored and cared for by any sort of pro- 
tection. At this point we are confronted by a phase that needs 
explanation. The life-history of every plant may be consid- 
ered a cycle, from the spore which produced it round to the spore 
which it produces. The cycle is traveled continuously without 
cessation, except at some one point, which is known as the “resting 
stage.” Every plant, in the life cycle referred to, must, at some 
point, pass through a resting stage, in which condition the plant 
activities lie dormant, as if to gather strength for the rest of the 
journey. This stage must always be a protected one, protection 
which not only shuts out adverse external conditions at a time of 
low vitality, but prevents response to favorable ones until after a 
certain lapse of time. In the groups already considered, this resting 
stage occurs at the spore phase. The protection provided is simply 
a thick heavy wall about the spore itself; and in this condition the 
plant exists for a time and then runs its cycle, round through parent 
form to spore again. To pass through the resting stage at the spore 
phase is characteristic of a low type. In the third group the 
resting stage is pushed gradually forward, until the sex-spore 
becomes, not a rather permanent phase, but simply one of the 
transient phases, the resting stage occurring after the spore has 
developed subsequent structures. , 
