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 akene. 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 have devel- 

 oped. The simplest plants that are now living have doubtless 



