Fig 1.
Bidirectional flowering in the head of Dipsacus fullonum.
(a) The middle flowers start flowering. (b, c) Flowering sequence proceeds both upward and downward
Fig 2.
Head development in Dipsacus fullonum.
(a) Vegetative shoot apical meristem (SAM). (b) Naked head meristem. (c) First flowers are initiated in contact parastichies; some parastichies are delayed compared to adjacent ones (arrows). (d) Elongation and expansion of the head meristem and acropetal formation of parastichies. (e) Completion of floral initiation. Note the spindle-like shape of the meristem and the smaller size of the basal bracts (arrow) compared to the middle ones (arrow). (f) Mature head with all bracts removed except at the top. (g) Details from (f) showing the larger size of the middle flower primordia compared to the basal and apical ones (same scale). (h) Flower organogenesis starts in the middle flowers and proceeds towards both ends. (i) Details from (h) showing the promotion of the development in the middle flowers (same scale). IB = involucral bracts; L = leaf; rL = removed leaf; SAM = shoot apical meristem.
Fig 3.
Quantitative changes during head development.
(a) Head diameter at the basal and middle part in three developmental stages (S1, S2, S3); meristem expansion starts earlier in the middle part and shows a higher rate compared to the basal part. (b) Comparison of head diameter and flower primoridum size in the basal, middle and apical zones of the heads in stage S3. The expansion of the head in the middle part is paralleled with the promotion of the middle flowers (n = 10 per developmental stage). For each measured trait, characters that share the same letter do not differ significantly on the basis of Tukey’s test (p<0.05).
Fig 4.
Parastichy formation in Dipsacus fullonum.
(a) Regular pattern. (b) Parastichies unite at the tip of the meristem. (c) Additional contact lines appear after meristem expansion.
Table 1.
Phyllotactic patterns in Dipsacus fullonum (n = 40).
Fig 5.
Schematic diagram illustrating the putative effects of early and late meristem expansion on flower initiation.
(a) The head meristem fractionates flower primordia in an acropetal direction (white lines). (b) If the geometry of the head does not change, regular parastichies appear. (c, d) If the meristem expands in its middle part, spatial conditions change. Depending on the time of expansion relative to the time of primordia initiation, already existing primordia are promoted (c) or additional parastichies are formed (d).