Are Notobasis and Onopordum Cheating Ants?

Published: October 15th, 2009 | Updated: 17/01/15

The leaves and young stems of Notobasis syriaca resemble plants of S. marianum (Fig. 8.4.1) in appearance. The achenes resemble each other in size, hardness, and smoothness (Fig. 8.4.2). N. syriaca has no oily body, however plants of N. syriaca are often found near S. marianum by the ant nests. Those searching for explanations and titles in ecological research will be studying here cheating, disguise, and resource-saving of Notobasis compared to Silybum. According to these ideas, which await experimental proof, Notobasis may take advantage of the connection formed between Silybum and the ants, and “camouflages” itself as a Silybum.

I carried out preliminary experiments in comparing the preference of ants for the two; there was some preference for the Silybum, but since then I have not had the time to carry out serious investigations. When guiding teachers in the field, I suggested giving their pupils research projects including Notobasis and Silybum. I later heard that these projects were very successful, but not one teacher sent me a report on such a project. If any of my readers feel that I have tried to provoke guilt feelings in them in order to receive such a report – this is correct, and I am ready to learn from your experience.

Fig. 8.4.1: Notobasis syriaca which strongly resemble Silybum marianum in achenes and vegetative parts morphology.

Fig. 8.4.2: Achenes of Silybum marianum (1. with oily body, 2. without oily body,) and of Notobasis syriaca (3).

In Israel Onopordum blancheanum is limited to the basalt plateaus of the Golan and eastern Upper Galilee. The plant was described in 1942 as a subspecies of a more common species: Onopordum carduiforme Boiss. subsp blancheanum Eig. In 1972, during my postdoctoral studies I was involved in the preparation of the genus Onopordum for the Flora of Turkey: (Danin, A. 1975. Onopordum. In: P.H. Davis (ed.) Flora of Turkey, Edinburgh University Press 5: 356 369.) Later, when applying several principles for the recognition of Onopordum species in Israel, I raised the taxon of the basalt lands to a species level (O. blancheanum, in Israel J. Bot. 37: 57 59.; 1988). Eventually, I was happy to discover the affinity of O. blancheanum to ant nests despite having no elaiosome (Figs. 8.4.3-8.4.5). If this subject is of interest to future ecologists, perhaps they will study the ties between these thistles and the ants.

Fig. 8.4.3: Onopordum blancheanum with flowering heads in a circle with a nest of harvesting ants in its center.

Fig. 8.4.4: Leaf rosettes of Onopordum blancheanum in a circle with openings of a nest of harvesting ants at its center.

Fig. 8.4.5: Inflorescences of Onopordum blancheanum.

Carduus near my home

Throughout my increasing interest in oily bodies and their being a bait for ants, I studied all the illustrations of plants from the Compositae (sunflower family – Asteraceae) in Flora Palaestina part 3. I searched for achenes that resemble the achene morphology of S. marianum (Fig. 8.3.3). and was surprised to find that several Carduus species have elaiosome (Fig. 8.4.6). A not less surprising finding was the growth of Carduus argentatus at the joining point of my garden wall and the asphalt sidewalk near it. It is evidently associated with the ants living there.

I did not make myself sit nearby and watch ants moving Carduus achenes, but I am confident that the following sequence takes place there: plants bloom, achenes ripen (Fig. 8.4.8), its pappus bristles attach to each other (as in Fig. 8.4.6, 8.4.8, and 8.4.9 on their right) and open when dried, to the state shown in Figs. 8.4.8 and 8.4.9 (left). Drying out of the pappus from the status shown in Fig. 8.4.9 (right) to that of Fig. 8.4.9 (left) leads to the achenes slipping off from the inflorescence’s receptacle. Now they are dispersed a short distance by wind, and ants transfer the achenes landing in their vicinity into the nest. Achenes with an elaiosome serve as a reward to the ants pulling them into the nest. Carduus species also have hard, smooth achene coats; the ants dispose of the achene and keep its oily body to feed the youngsters in the nest. In this way an increasing reservoir of achenes accumulates in the fissure between the side walk and the base of the garden wall, as dealt with in the matter of Fumaria capreolata.

Fig. 8.4.6: Achenes of Carduus argentatus. On the right an achene with a closed, wet pappus; nearby 2 achenes, the pappi of which were removed. Their oily-body is prominent. On the left, an achene under greater magnification.

Fig. 8.4.7: Carduus argentatus blooming.

Fig. 8.4.8: Flowering heads of Carduus argentatus at the stage of achene dispersal. On the right an inflorescence with closed pappus as in Figs. 8.4.6 and 8.4.9 on the right.

Fig. 8.4.9: A longitudinal section of a closed inflorescence of Carduus argentatus (on the right). The involucral bracts cover the achenes with closed pappi. When drying out, the pappus bristles diverge, thus removing the achene from the receptacle.