Date of Award:


Document Type:


Degree Name:

Master of Science (MS)



Department name when degree awarded

Biology, and The Ecology Center

Committee Chair(s)

Ivan G. Palmblad


Ivan G. Palmblad


B. E. Norton


Jim MacMahon


Weedy species, as they occupy diverse and unpredictable environments, are expected to possess considerable variability in numerous characteristics, including germination. The visually dimorphic achenes of Heterotheca grandiflora Nutt. (Asteraceae) were tested for potentially adaptive differences in their germination, the establishment of the resulting seedlings, and the relative production of the two achene types.

Achenes produced by the disc florets are adapted for dispersal and rapid germination while the ray florets produce smaller achenes lacking a dispersal mechanism, but possessing a more sophisticated physiological system capable of regulating germination. Ray achene germination rates were significantly influenced by different soil surfaces while disc achene germination rates were not. Further, ray achenes {protected by a thicker seed coat) may remain in enforced dormancy until conditions become appropriate for germination.

Under continuously favorable conditions, seedlings from early germinating disc achenes will dominate a mixed stand and suppress those from later germinating ray achenes. An initially unpredictable environment will eliminate disc seedlings and permit the more dormant ray achenes to germinate in an environment of reduced competition.

Limited tests suggest a greater proportion of the smaller and energetically cheaper ray achenes may be produced when resources are scarce due to drought stress.

These germination characteristics are only one aspect of this taxon's developmental plasticity. The ability to act as an annual or biennial, flower in the rosette stage, and produce seed in great numbers under favorable conditions have all acted together with the germination characteristics to help preadapt this plant for its weedy spread from its original southern California range.



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