APPENDIX 201 
16. The pistils are often united with each other. Here all stages 
of union occur, from a slight union at the base to complete fusion 
to the stigma. When the ovaries are united they form a compound 
ovary. This can be distinguished from a single simple ovary by 
having several cavities or cells or by having several placentae. This 
will be seen in a cross-section. Often they can be distinguished by 
having several styles or stigmas. 
17. The floral structures of one series often unite with those of 
adjacent series. Thus the stamens often unite with the corolla 
especially in gamopetalous flowers as the morning glory or the 
phlox. 
18. In many flowers the lower part of the receptacle is ex- 
panded into a disc or hypanthium on the edge of which the perianth 
and stamens are borne. This structure is well shown in the straw- 
berry blossom. This disc may grow upwards into a cup or tube as 
in the plum or cherry. In these it disappears after the blossom 
fades, but in the rose it is permanent and forms the flesh of the 
hip and in the apple where it unites with the pistils and forms 
the outer part of the flesh of the apple. In other plants, as in the 
evening primrose, it forms a tube extending beyond the ovary. When 
the hypanthium reaches to the top of the ovary or beyond and is 
adnate to it, the ovary becomes inferior, and the flower epigynous. 
19. When all the members of each whorl of floral organs are 
alike or nearly so the flower is regular; if they differ from each 
other, one side of the flower being markedly different from the other 
it is irregular. 
20. The arrangement of the flowers i. e. the inflorescence is. fre- 
quently referred to in the keys. The names of the common flower 
clusters will be given in the glossary. 
21. After fertilization the ovary developes into the fruit. The 
walls of the ovary form the pericarp, which consists of two layers, 
the inner is the endocarp the outer the exocarp. These are often 
quite different, for instance in the plum the exocarp is juicy forming 
the flesh of the fruit while the endocarp is horny and forms the shell 
of the pit. 
22. Fruits which do not open are indehiscent, while those that 
open at maturity to allow the seeds to escape are dehiscent. Most 
dehiscent fruits split lengthwise into valves or teeth. If they have 
several cavities they may open at the middle of the cavities or at 
the dividing walls between them. The former is called loculicidal 
and the latter septicidal dehiscence. If the fruit breaks regularly 
crosswise, the top coming off like a lid, it is circumscissile. 
23. A leaf may have three distinct parts, the blade, the stalk, 
or petiole and the stipules. The blade is the expanded green- part 
forming the main portion of the leaf. Leaves with more than one 
blade are compound. The stipules are small appendages on the 
sides of the petiole at its base. They are frequently wanting, or 
represented only by glands. The petiole may also be wanting, the 
leaf is then sessile. 
24. In the grasses and sedges the lower part of the leaf sur- 
rounds the stem and is called the sheath, the free part of the leaf 
is the blade. In the grasses where the blade joins the sheath an 
appendage called the ligule usually extends upwards around the 
stem. 
