304 INTRODUCTION TO BOTANY 
ovary wall ripens as a close-fitting, thin coat about the seed, 
and the kind of fruit thus formed is called the achene. In the 
stony fruits, as in the peach and apricot, the ovary wall divides; 
the inner part produces the hard covering about the seed, and 
the outer part produces the pulpy flesh. In the apple the calyx 
is joined to the wall of the ovary, the seeds are inclosed in the 
ovary cavities, the ovary wall ripens, thus becoming the core 
of the apple, and the calyx ripens into the greater part of the 
fruit. A transverse or a longitudinal section of an apple or pear 
will usually enable one to determine what part of the fruit is 
the ripened calyx and what part is the ovary wall. 
In some cases whole clusters of flowers and the modified 
portions of the stem upon which they grew may ripen into a 
single fruit; or the pistil may grow while the seeds are de- 
veloping, as in the beans and peas, so that the ripened pod is 
hundreds of times as large as the pistil was when fertilization 
took place. 
The distribution and germination of seeds was fully dis- 
cussed in previous chapters, and if those chapters are briefly 
reviewed at this time, it will be found helpful. 
289. Evolution of plants. The four great divisions of the 
plant kingdom, and the most important classes of these divi- 
sions, have been discussed. It must have been apparent to 
most students that constant increase in the complexity of 
plants was encountered as we passed from lower to higher 
groups. This increase in complexity appears in the nutritive 
parts of plants and in the parts that have to do with repro- 
duction. The process of gradual development from simple to 
complex is the process of evolution. Indeed, there may also 
be evolution in the opposite direction, as occurs when, through 
long and constant changes, simple forms are derived from 
complex ones. Usually evolution is thought of as having 
to do with increase in complexity rather than with decrease. 
The oldest plants of the earth were very simple, and from 
them, in one way or another, more complex ones haye deyel- 
oped. The simplest plants that are now living have doubtless 
