ABORTION 
3i 
Abortion and Suppression of organs are other 
common phenomena. In a certain place in a certain plant, 
comparison with related forms often shows that we should 
expect to find a certain organ, say a stamen; instead 
of this we find a small functionless rudiment due to arrested 
development (abortion), or nothing at all ( suppression ). 
These phenomena may be both onto- and phylo-genetic, i.e. 
may occur in a single individual without being necessarily 
hereditary, or regularly and hereditarily in all individuals 
of a species. The importance of aborted (also called 
vestigial or midimentary) organs in morphological and 
taxonomic study is considerable, as affording useful clues 
in phylogeny. The abortion or suppression may be due 
to loss of function, disadvantageousness of the organ, 
correlation, mutation, or other causes. 
Symmetry. It rarely happens that an organ (or 
plant) is asymmetrical , i.e. so constructed that it cannot be 
divided in at least one direction into two halves one of 
which is the counterpart or reflection of the other (cf. leaves 
of Begonia, flowers of Valerianaceae). Complete spherical 
symmetry is rare except among very lowly forms of life, but 
what is termed radial (or better multilateral ) symmetry, 
such that any plane passing through the axis of growth 
of an organ and through one of the lateral branches of that 
organ, divides it into complementary halves, is common, 
e.g. in roots, erect stems, many flowers (actinomorphic, cf. 
diagram in Rosaceae) &c. Another frequent type of sym- 
metry is bilateral, in which only one axial plane divides 
the organ evenly, e.g. in many flowers (zygomorphic, cf. 
diagrams in Labiatae, Orchidaceae), leaves of Iris, &c. 
A special case of this is dorsiventral symmetry, in which 
the organ has an upper or ventral side different from the 
lower or dorsal, as in most leaves, most zygomorphic flowers, 
many horizontal shoots, &C . 1 
Classification of Organs. General comparison 
shows that certain structural characters are almost universal 
among the higher plants, and therefore to be regarded, on 
the principles indicated above, as derived from a very 
1 Goebel, Organography of Plants, Engl, ed., p. 65 ; Willis, 
Podostemaceae (dorsiventrality) in Ann. Perad. I. 1902, p. 434, 
