Types of Floret in Cotula
There are three main types of floret found in Cotula: female, hermaphrodite and male.
The female florets: (Fig. 2) are present in one or more outer rings in all species (other than the dioecious ones), with the exception of some South African species where they have been lost through abortion and the heads are composed entirely of hermaphrodite florets. (These are the homogamous heads mentioned by Cheeseman (1925), but there are no species of Cotula with this type of head in New Zealand.) There are two kinds of female floret, as has been noted in the preceding section. There is the type which occurs in the sections Eucotula and Strongylosperma, which is long-stalked, with its corolla so reduced as to be almost non-existent, and its gland-covered ovary with two broad spongy wings, the short bifid stigma being sunk in a notch between these wings. Two species in New Zealand, Cotula coronopifolia (Fig. 2A) and C. australis have this type of female floret. The contrast between this type of floret and the Leptinella female floret, which occurs in all the other New Zealand species, has already been mentioned, the principal difference being the presence of the corolla in Leptinella. In some species this corolla is a small tube inflated at the base, and narrowed into a minutely four-toothed mouth. This occurs, for example, in Cotula haastii (Fig. 2c), C. pectinata, C. dioica and C. squalida. In other species, Cotula atrata and C. pyrethrifolia, the corolla is not so compressed and is obviously lobed at the mouth.
The stigma in these female florets is the bifid stigma characteristic of female florets throughout the Compositae, though at times the style arms are extremely short, as is noted by Kirk for several of the species. Bentham (1873) writes of the styles
Text-fig. 1.—Fig. 1—A. Section through part of head of C. coronopifolia (Eucotula) × 20. B. Section through part of head of C. australis (Strongylosperma) × 20. G. Section through part of head of C. Haastii (Leptinella) × 15. Fig. 2.—A. Female floret of C. coronopifolia × 30. B. Hermaphrodite floret of C. coronopifolia × 30. C. Female floret of C. Haastii × 30. D. Male floret of C. Haastii × 30.
Fig. 3—A. Diagram of L.S of male floret of C. Haastii. B. Diagram of L.S. of hermaphrodite floret of C. australis.
of the female florets of the Compositae, “These styles of the female florets are uniformly divided into two equal, more or less stigmatic branches, glabrous without and papillose inside, which may occasionally vary in length or thickness, but only slightly so, and very rarely, as far as I have been able to observe, so as to give any but a very slight generic clue.” For this reason the description given by Petrie (1885) of the stigma of the female floret of Cotula Goyeni seemed to point to a remarkable exception. This description is as follows: “Style crowned by a thin disc-like flattened stigma in both female and hermaphrodite flowers.… In some specimens the stigmatic disc shows traces of a division into two lobes but I have seen none with anything like two branches to the style.… If this peculiarity should prove constant in the present species, and it should continue to be regarded as a Cotula, the character of the genus as now formulated will require modification.” Heads of the type specimen from Petrie's herbarium were examined, and it was concluded that although the style arms were very small and compressed for a female Composite stigma, the division into two lobes was always present and that this stigma was quite distinct from the club-shaped, flat-topped stigma of the male floret to which Petrie had compared it.
The hermaphrodite florets: of Cotula coronopifolia (Fig. 2B) and C. australis —belonging to the Eucotula and Strongylosperma sections respectively—have a tubular corolla, broadening to a four-lobed mouth and the ring of anthers surrounding the stigma are those typical of the tribe Anthemideae, having no tails. The stigma is bifid, of the type which Bentham termed the Senecio style, and he notes that it occurs in the Anthemideae. He describes this style as having flattened branches, narrow and recurved, with marginal stigmatic series which reach to the extremity, which is truncate and fringed with hairs.
The male florets: resemble the hermaphrodite florets of the other two sections in so far as they have a similar corolla and anthers. They differ, however, in the character of the stigma which is discoid and undivided, with hairs round the rim, but no stigmatic papillae (Fig. 2D). To quote Bentham once more: “In all tribes which admit of central sterile hermaphrodite florets (Asteroideae, Inuloideae, Helianthoideae, Helenoideae, Anthemideae, Senecionideae, Calendulaceae and Arctotideae) the styles of these sterile florets are for the most part filiform or slightly clavate, undivided or with slender erect branches, very papillose or hirsute outside, without stigmatic series inside, and very similar in all the different tribes where they occur.” The stigma of the male floret, then, has retained what Bentham has termed its “principal and often sole function of sweeping the pollen out of the antheral tube” but (without a stigmatic surface) it has lost the function of allowing the pollen grains to germinate.
(a) The abortion of the ovule in the male floret.
Although the disc shaped stigma in the male floret was found to have no stigmatic papillae and had therefore lost the function of allowing the pollen to germinate, it was possible that the ovary beneath this sterile pistil might still be functional, despite the fact that it invariably appeared to be minute. The development, therefore of the ovary of Cotula Haastii, in which the disc florets set no seed, was investigated and compared with that of the hermaphrodite disc florets of Cotula australis on which seed is set.
(i) Materials and method.
Whole heads of Cotula Haastii and C. australis were killed and fixed in formalinaceto-alcohol. Dehydration was carried out by passing through a graded series of tertiary butyl alcohol solutions (Johansen, 1940). The heads were embedded in paraffin wax and microtome sections were cut longitudinally through them. Sections were stained with Heidenhain's iron haematoxylin.
(ii) Comparison of the Ovary in Cotula Haastii and C. australis.
The development of the ovary is the same in the two species up to the formation of the cavity into which the ovule normally forms. In Cotula Haastii, however, no ovule develops, whereas in Cotula australis an ovule develops normally. If sections of Cotula Haastii and C. australis at similar ages are compared, an empty cavity can be seen in the ovary of C. Haastii below the base of the style (Fig. 3A), whereas in C. australis the ovule can be seen protruding into this cavity (Fig. 3B). Male florets of Cotula Haastii at a later stage than this show that the ovary has enlarged very little more, and that the cavity of the ovary is almost obliterated.
(b) The development of the stigma of the male floret.
Sections of immature florets showed that the stigma of the male florets is at first two lobed as is the hermaphrodite stigma (Cf. Figs. 3A and B). At a later stage in development, however, the two lobes coalesce to form the mature undivided stigma. The existence of sweeping hairs of a similar type at the ends of both the hermaphrodite stigma and the male stigma would seem to suggest that if the two lobes of the hermaphrodite stigma, for some reason joined together during their development, and in consequence lost their stigmatic papillae, the resulting stigma would resemble the male stigma in appearance. It may be that a change of this nature was responsible for the type of stigma which occurs in the disc florets of two Australian species investigated, belonging to the section Strongylosperma—Cotula alpina and C. reptans. These have the Eucotula-Strongylosperma type of female floret combined with the Leptinella type of male disc floret with a flat-topped stigma, instead of the hermaphrodite floret with bifid stigma which might be expected on analogy with C. australis, another member of this section. The species of Strongylosperma are therefore, not uniform, some being close to Eucotula—C. australis, and others approaching Leptinella—C. alpina and C. reptans. This combination of floret types suggests that Strongylosperma is intermediate between Eucotula and Leptinella. (Table I.)
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|Female apetalous||Stipitate. Corolla absent. Ovary winged, notched at the top. Bifid stigma sunk in notch.||Eucotula Strongylosperma|
|Female, corolla present||Sessile or shortly stipitate. Corolla of two layers, inflated at the base, narrowed at the mouth into small teeth, sometimes tubular and lobed. Ovary hardly winged.||Leptinella|
|Hermaphrodite||Shortly stipitate. Corolla tubular, broadening to a four-lobed mouth. Stigma bifid, achenes smaller than in female with narrower wings||Eucotula Strongylosperma|
|Male||Sessile. Corolla tubular, broadening to a four-lobed mouth. Stigma discoid and undivided without stigmatic papillae. Achenes minute and abortive.||Leptinella Strongylosperma (Cotula alpina) (Cotula reptans)|