FLOWER AND FRUIT 151 
swollen. In some eases, as in the strawberry, black- 
berry, and apple, it eventually forms an important 
part of the fruit. 
The Sepals, which form the lowest whorl of the 
flower, and vary in number according to its kind, are 
collectively known as the calyx. They are the most 
like foliage leaves, for they are generally green, and 
are thus fitted to help in the work of photosynthesis. 
The sepals may be quite distinct from one another, as 
in the buttercup, in which case the calyx is termed 
aposepalous (Gk. apo. away) or they may be united 
to form a tube, as in the bean and apple, when the 
ealyx is said to be synsepalous (Gk. syn together). 
At this stage a word or two on evolution will not 
be out of place. It is reasonably certain that the 
plants that exist on the earth to-day have had common 
ancestors. Slight variations, arising from time to time 
in individual descendants, have, in the course of ages, 
resulted in the vast variety of species that at present 
covers the earth. Careful investigation leads to the 
conclusion that certain types of plants and certain 
forms and arrangements of parts are more primitive 
than others. We shall find that there are plants which 
deviate widely from the most primitive type in all 
respects. In these, there has been progressive develop- 
ment or evolution in all directions... In others again 
the development has been only partial and frag- 
mentary. Certain organs may have become highly 
differentiated, while others have retained their 
primitive form and arrangement. Here evolution has 
been restricted to certain lines. As an example of 
progressive development may be mentioned the union 
of the sepals to form a tube, as seen in the bean. This 
may be contrasted with the more primitive free con- 
dition of the members forming the calyx of the 
buttercup or pansy. Other cases will be dealt with as 
they arise. 
