Chaptalia texana Reproduction: Chasmogamy & Apomixis
by Bob Harms  email-here

Type 1 Seedlings, 26 May — Achenes from NNN April

Context: The sole pollen source for sexual reproduction is from the small set of central florets.

Evidence for Open–Head (Type I) Chasmogamy

March and early April heads aborted and dried. Nodding is not normal for mature heads.

The earliest (March — early April) open–heads did not produce mature/viable achenes. During that period I noted that the anther tubes of the central florets were prominently exerted, but were not opening, and the style branches were not visible. Later, in mid April, the style branches were exerted, and from that time many heads matured to produce viable achenes. On April 23 only 5 buds were found; none of these produced mature heads, although one continued to develop through May 15.

Achenes collected in mid April were collected and planted in a container. The rate of germination seems quite high. (Photo at top of this page)

I thus conclude that the pollen from the central florets is essential for fertilization. No support for self–pollination (even within the central florets, with both stigmata and pollen present) nor apomixis was found.

Apomixis with Closed–Head Types (II, III)

No closed heads failed to develop a large set of achenes even though the central florets (at least at early anthesis) were often covered by pappus bristles. And even when later in anthesis the anthers were exerted, the eligulate pistillate florets (over 60% of a head's florets) generally remained tightly covered by pappus, with styles that required microscope dissection to locate.

Type II head with well-developed achenes, April 27. [click to enlarge]

Type III head with well-developed achenes, April 8. [click to enlarge]

In my opinion, apomixis would appear to be the most plausible means capable of accounting for the high number of fertile achenes in the closed–head florets.

Possible Chasmogamy with Closed–Head Types (II, III)

Inspection of the achenes in the type III (April 8) image above shows that not all achenes are equally well developed. Those in the center that are less well developed, repeated below with red lines, belong to perfect central florets (which can be seen through the pappus wall).

Although the central florets do develop apparently well–formed achenes, a relatively large number of them do not. A set of 24 central–floret achenes collected at dispersal gave the following results: 8 (below left) were clearly well developed; 7 (marked with green 'x') were smaller, but possibly ok; 9 (red 'x') did not seem to have developed.


This was not typical for the eligulate pistillate florets, only a few of which had aborted.

I am left to speculate that perhaps the perfect central florets, unlike the others, develop sexually (whether self–pollinating or not). This would give the plant an adaptive advantage, in contrast with clonal apomictic reproduction. I might also note in this regard that for a large number of plants in the type II/III areas there was no noticeable variation in head morphology, whereas other plants from other areas (types 'II-III' and IV) were quite different.

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