Figs. 230, 231.
At first it seems that flowers thus placed should alone be radial; but further consideration discloses conditions under which this type of symmetry may exist in flowers otherwise placed. Remembering that the radial form is the primitive form—that, morphologically speaking, it results from the contraction into a whorl, of parts that are originally arranged in the same spiral succession as the leaves; we must expect it to continue wherever there are no forces tending to change it. What now must be the forces tending to change it? They must be forces which do not simply affect differently the different parts of an individual flower. They must be forces which affect in like contrasted ways the homologous parts of other individual flowers, both on the same plant and on surrounding plants of the same species. A permanent modification can be expected only in cases where, by inheritance, the effects of the modifying causes accumulate. That they may accumulate the flowers must keep themselves so related to the environment, that the homologous parts may, generation after generation, be subjected to like differentiating forces. Hence, among a plant’s flowers which maintain no uniformity in the relations of their parts to surrounding influences, the radial form will continue. Let us glance at the several causes which entail this variability. When flowers are borne on many branches, which have all inclinations from the vertical to the horizontal—as are the flowers of the Apple, the Plum, the Hawthorn—they are placed in countless different attitudes. Consequently, any spontaneous variation in shape which might be advantageous were the attitude constant, is not likely to be advantageous; and any functionally-produced modification in one flower, is likely to be neutralized in offspring by some opposite functionally-produced modification in another flower. It is quite comprehensible, therefore, that irregularly-branched plants should thus preserve their laterally-borne flowers from undergoing permanent deviations from their primitive radial symmetry. Fig. [230], representing a blossoming twig of the Blackthorn, illustrates this. Again, upright panicles, such as those of the Saxifrage exemplified in Fig. [231], and irregular terminal groups of flowers otherwise named, furnish conditions under which there is similarly an absence of determinate relations between the parts of the flowers and the incident forces; and hence an absence of bilateralness. This inconstancy of relative position is produced in various other ways—by extreme flexibility of the stems, as in the Blue-bell; by the tendency of the peduncles to curl to a greater or less extent in diverse directions, as in Pyrola; by special twistings of the peduncles, differing in degree in different individuals, as in Convolvulus; by unusual laxity of the petals, as in Lythrum. Elsewhere the like general result arises from a progressive change of attitude, as in Myosotis, the stem of which as it unfolds causes each flower to undergo a transition from an upward position of the mouth to a lateral position; or as in most Cruciferæ, where the like effect follows from an altered direction of the peduncle.
There are, however, certain seemingly-anomalous cases where radial symmetry is maintained by laterally-placed flowers, which keep their parts in relative positions that are tolerably constant. The explanation of these exceptions is not manifest. It is only when we take into account certain incident actions liable to be left unremembered, that we find a probable solution. It will be most convenient to postpone the consideration of these cases until we have reached the general rule to which they are exceptions.
§ 234. Transitions varying in degree from the radial towards the bilateral, are common in flowers that are borne at the ends of branches or axes which are inclined in tolerably constant ways. We may see this in sundry garden flowers such as Petunia, or such as Isoloma and Achimenes, shown in Figs. [232 and 233]. If these plants be examined, it will be perceived that the mode of growth makes the flower unfold in a partially one-sided position; that its parts of attachment have rigidity sufficient to prevent this attitude from being very much interfered with; and that though the individual flowers vary somewhat in their attitudes, they do not vary to the extent of neutralizing the differentiating conditions—there remains an average divergence from a horizontal unfolding of the flower, to account for its divergence from radial symmetry.
Figs. 232, 233.
We pass insensibly from forms like these, to forms having bilateral symmetry strongly pronounced. Some such forms occur among flowers that grow at the ends of upright stems; as in Pinguicula, and in the Violet tribe. But this happens only where, in successive generations, the flower unfolds its parts sideways in constant relative positions. And in the immense majority of flowers having well-marked two-sided forms, the habitual exposure of the different parts to different sets of forces, is effectually secured by the mode of placing. As illustrations, I may name the genera—Orchis, Utricularia, Salvia, Salix, Delphinium, Mentha, Teucrium, Ajuga, Ballota, Galeopsis, Lamium, Stachys, Nepeta, Marrubium, Calamintha, Melittis, Prunella, Scutellaria, Bartsia, Euphrasia, Rhinanthus, Melampyrum, Pedicularis, Linaria, Digitalis, Orobanche, Fumaria, &c.; to which may be added all the Grasses and all the Papilionaceæ. In most of these cases the flowers, being sessile on the sides of upright stems, are kept in quite fixed attitudes; and in the other cases the peduncles are very short, or else stiff enough to secure general uniformity in the positions. A few of the more marked types are shown in Figs. [234 to 241].
Figs. 234–241.