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The Evolution of the Inflorescence. by J TEE EVOLUTION OF THE INFLORESCENCE. 511 The Evolution of the Inflorescence. By J. PARKIN,M.A., F.L.S., Trinity College, Cambridge*. (PLATE18, and 9 Text-figures.) [Read 18th December, 1913.1 CONTENTS. Page Introduction .......................................... 511 Historical ............................................ 513 The General Character and Biological Significance of the Inflorescence ...................................... 518 The Solitary Terminal Flower and the origin from it of the sinlplest Inflorescence-The Dichasium .............. 621 The Evolution of Racemose Inflorescences from the Pleio- chasium or Panicle ................................ 638 Continuous Dichasia, Monochasia, and Sympodial Cymose Inflorescences .................................... 545 The Origin of Solitary Axillary Flowers .................. 648 Intercalary and Pseudoterminal Inflorescences ............ 556 Summary ............................................ 559 I. INTRODUCTION. TEE study of the Inflorescence from the evolutionary point of view has been strangely neglected. The treatment of this subject appears hardly as yet to have advanced beyond the limits of pure description. Botanical text-books have been and are still in the main content with a formal treatment of the Inflorescence. In these the various types of flower-groupings are at the outset sharply divided into the two classes, racemose and cyinose ; then follow con- cise descriptions of the various kinds contained in the two categories. No attempt is made to connect racemose with cymose inflorescences, nor to trace the evolution of flower-clusters from some simple arrangement, as is to be found in the solitary terminal, or even in the single axillary flower. The account of the Inflorescence in one botanical text-book appears usually as a ,&Ti& copy of what occurs in an earlier work of a similar kind. ‘Phe subject, in fact, is dealt with in :i rather perfunctory fashion. * A brief exposition of most of the view expressed in this pzper was given before the Botanical Section of Ihe British Association at the Winnipeg Meeting (for Abstract 888 1‘ The Annual Report,” 1909, p. 662). Since then the aiithor has not seen reason to depart essentially from any of the views there given. LINN. SOURN.-BOTANY, VOL XLII. 2N 512 IIR. J. PARKIN ON THE Thus the study of flower-l)ranching and arrangement, which at the best is perhaps not a particularly attractive subject, remains duller than it other- wise niight be through this “ cut and dried ” tnnnner of treatment. Heiice an effort is here made to give these “ dry bone$ ” of descriptive morphology “ flesh,” or at any rate some semblance of I‘ flesh.” In this paper an attempt is niade not only to connect together cymose and racemose types cf inflorescences, but also to trace all flower-groupings from one primitive source, viz. :-a leafy shoot bearing a solitary terminal flower. From such a shoot, simple cymose flower-clusters will be shown to arise first; and then from these, will be traced the evolution of racemose forms. The raceme or spike originating thus, is consequently by no means an early form of inflorescence, but one which has had a cymose ancestry. Even the solitary axillary flower is shown to be capable of derivation indirectly from the terminally borne flower. Thus is attempted the evolu- tionary connection of all flower-arrangements found in Angiosperms *. In this paper a new term is introduced for that kind of‘ flower-cluster, the main axis of which continues to grow vegetatively after emitting the flowers. The name intercalary is proposed for this. And, finally, it is shown that such intercalated flower-clusters may give rise to what are here termed pseudo- terminul inflorescences, through the arresting of the vegetative continuation of the main axis. Such an inflorescence, in regard to the order in which the individual flowers composing it bloom, appears identical with a racemose cluster which has arisen froma cymose panicle. Hence it is pointed out that racemose inflorescences (racemes, spikes, umbels, etc.), may arise in two totally different ways, viz. :-from cymose groupings or from intercalary inflorescence^. I-tensons are given for regarding these two kinds of inflores- cences as distinct in origin and tnorphological value. The one, the inter- calary, lins merely resulted from the segregation of selmrate flower-bearing shoots ; \\ hile the other, the cymose, has arisen by the production of‘ some- thing essentially new in the history of the plant, namely, additional flowers alongside the originally solitary terminal flower. Most inflorescences probably belong to the Iiitter category. The recognition, however, of the former type is looked upon as one of the novel features of this paper. Most of the evidence for the views thus briefly formulated, has been culld from au examination of fresh plants ; resort has been had to some extent to herbarium specimens ; but very little use has been made of systematic descriptions or drawings of plants. Descriptive remarks applicable to inflorescences or the way the flowers are borne, to be found in systematic and * It might be more correct to write here, “ Dicotyledons,” as this paper is almost wholly restricted to (t consideration of tho inflorescences of this group. The Monocotyledons in the author’s opinion are a I early ofshoot from the Dicotyledons, much moditied, and so of less me in studying the evolution of flower-grouping. EVOLUTION OF THE INFLORESCENCE. 513 other botanical works, are often of :i vague character, SO that little reliance can be placed upon them as to the true nature of the flower-arrangement. This may not be surpriging. Without an evolutionary basis for the classifi- cation of the inflorescence, the connecting or intermediate forms, the ones of especial value in this paper, are apt to be tAe least clearly described. Hence a more complete understanding as to how flower-cluster4 have arisen, and as to how the larious kinds have evolved one from another, would assist the systematist in the formulation ot his descriptions ; and then these latter would convey to the morphologist, seeking data, clearness as to how the flowers are borne, and as to the order in which they bloom. As the author has, therefore, not tliought fit to utilise to any extent information respecting the Inflorescence to be obtained from systematic descriptions of genera and species, the evidence bronglit forward in the succeeding pages may appear less complete than it otherwise might. If systematic descriptions could have been used, then the gaps in the study of the genera dealt with could have been readily filled. The examination in the fresh state of all the species known of a fairly large genus is a matter both of time and of difficulty. It seemed hardly desirable to postpone the publica- tion of the paper till this had been done for the various genera here con- sidered ; especially as the evidence has been drawti from examples widely scattered atnong the Dicotyledons, and does not depend solely on the investigation of one or two families. Also, at this stage it seemed fitting to elaborate a working hypothesis by the aid of which the subject can be further studied. Before closing this Introduction it may be well to draw attention to the following fact, which will be self-evident from a perusal of the main part of this paper, namely, that the evolution of inflorescences from solitary terminal flowers on the lines thus briefly indicated, has taken place independently in several families of Angiosperms. Though there inay have been slight differ- ences in detail, yet in each case the evolution has advanced by essentially the sitnie steps. N.B.-To avoid repetition in ihe explanations accompanying the diagram- matic text-figures, attention is called here to the fact that the individual flowers are represented by circles and the order of their expanpion by numerals (1, 2, 3, etc.). 11. HISTOKICAL. Before commenci~~ga short review of the literature bearing on this paper, it may be well to explain clearly the meaning herc attached to the term “ &LJlorescence.” LinnEeus * appears to have been responsible for the introduction of * Linnaeus, O., ‘Phil. Bot.‘, Stockholmiae, p. 112, 2761. 2N2 5 14 MR. J. PARKIN ON THE the term, which literally means the time of flower-bearing. It was applied by the great Swedish naturalist to the arrangement of the flowers on the axis or axes. Asa Gray * proposed the term “Anthotaxy,” to correspond with phyllotaxy, as preferable to that of “ Inflorescence ’’ for denoting flower-arrangement. However, the latter has persisted and is now in constant use in Botany, so little advantage would be gained by discarding it for another. Inflorescence has often been defined as the mode of branching of the floral axis. Van Tieghem t implies as much, though he proceeds to write that the term is sometimes given a concrete meaning by applying it to the floral group itself. Bentham and Hooker $, in their ‘British Flora,’ supply a comprehensive definition including both ideas. “The inflorescence of a plant is the arrangement of the flowering branches and OE the flowers upon them. An inflorescence is a flowering branch or the flowering summit of a plant above the last stem leaves, with its branches, bracts and flowers.” It seems futile to quibble over the question whether inflorescence means the mode of floral branching or the flower-group itself. Custom has sanctioned the latter meaning. When we speak of the inflorescence of a plant we employ the word as a concrete noun and apply it to the flower-cluster itself. It is in this sense that the word is used in this paper. The mode of branching of the flower-bearing axes of the inflorescence becomes one line for study. The sequence in which the individual flowers open is another. The character of the internodes, such as their proportional length or their suppression, is a third. The origin of flower-clusters and the phylogenetic relationship between the various kinds, that is to say the evolution of the inflorescence, furnish a further line of investigation, embracing as it were all the others.
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