Taxonomists, since and before the time of Linnaeus, have relied heavily on the value of characters inherent in the reproductive parts of vascular plants to serve as criteria of particular significance in their classifications. In the pteridophytes reproductive characters are associated in general with the sporangia and sex organs of the gametophyte; in the gymnosperms they are those of the pollen-producing and fruiting structures; and in the angiosperms are those of both the flowers and the fruiting structures. This last taxon is the most complex of all, and phylogentically is the most highly developed and advanced. For these reasons great diversity and complexity of reproductive characters are typical of its members. Since the objectives of taxonomy include identification and classification, it is necessary to understand the composition of the flower, of the theories purporting to account for the origin of its components, and of the presumed evolutionary levels represented by present-day types of these components. The disciplines of morphology, anatomy, and paleobotany have contributed the data and the conclusions responsible for these understandings.
Prerequisite to any basic understanding of the flower is an appreciation of the view that vascular plants structurally are evolved from or composed of different types of branch systems. As applied to flowering plants, and on the evidence adduced from the fossil record that terminal reproductive structures were present prior to the evolution of foliaceous leaves, it is probable that at least 3 types of branch systems have served as antecedents of present-day leaflike structures (Wilson, 1941).
These 3 types are:
1. a branch or cluster of branches bearing terminal sporangia from which the stamen is descended;
2. branches which became flattened (but were never primarily photosynthetic) to become the ovule-bearing carpels;
3. sterile branches or branch systems which became the leaves.
This postulate should be kept in mind when considering the conventional and very generally accepted theory (Eames, 1931) that "the flower morphologically is a determinate stem with appendages, and these appendages are homologous with leaves." In other words, flowers are homologous with leafy stems; the pedicel and receptacle of the flower are homologs of the stem axis, and the sepals, stamens, and pistil (s) are homologs of the leaves (i.e., the latter are appendages of the stem). If the quotation form Wilson is accepted, it becomes clear that although foliaceous leaves may be appendages of stems they may be presumed to have been evolved by the dorsiventral compression and lateral fusion of photosynthetic branches.
Prerequisite to any basic understanding of the flower is an appreciation of the view that vascular plants structurally are evolved from or composed of different types of branch systems. As applied to flowering plants, and on the evidence adduced from the fossil record that terminal reproductive structures were present prior to the evolution of foliaceous leaves, it is probable that at least 3 types of branch systems have served as antecedents of present-day leaflike structures (Wilson, 1941).
These 3 types are:
1. a branch or cluster of branches bearing terminal sporangia from which the stamen is descended;
2. branches which became flattened (but were never primarily photosynthetic) to become the ovule-bearing carpels;
3. sterile branches or branch systems which became the leaves.
This postulate should be kept in mind when considering the conventional and very generally accepted theory (Eames, 1931) that "the flower morphologically is a determinate stem with appendages, and these appendages are homologous with leaves." In other words, flowers are homologous with leafy stems; the pedicel and receptacle of the flower are homologs of the stem axis, and the sepals, stamens, and pistil (s) are homologs of the leaves (i.e., the latter are appendages of the stem). If the quotation form Wilson is accepted, it becomes clear that although foliaceous leaves may be appendages of stems they may be presumed to have been evolved by the dorsiventral compression and lateral fusion of photosynthetic branches.