Anisotomous dichotomy results from an unequal bifurcation of the original shoot apical meristem in Diphasiastrum digitatum (Lycopodiaceae).

PREMISE OF THE STUDY: Two types of dichotomy are recognized in Lycopodiaceae: isotomous (equal) and anisotomous (unequal). Anisotomous dichotomy (anisotomy) has been hypothesized to result from unequal growth of an equal bifurcation of the original shoot apical meristem (SAM). Diphasiastrum digitatum (Lycopodiaceae) exhibits anisotomy at various locations. We thus used D. digitatum to test this classic hypothesis about anisotomy.

METHODS: Transverse areas of original and derived SAMs of anisotomy exhibited by the rhizome and the vertical aerial vegetative stem were measured using scanning electron microscopy. The difference between half of the original SAM and one derived SAM in terms of transverse area were compared using paired t-tests.

KEY RESULTS: During the anisotomy exhibited by the rhizome SAM, 77.4% of the transverse area of the original rhizome SAM contributed to the derived rhizome SAM. During the first anisotomy exhibited by the vertical aerial vegetative stem SAM, 66.2% of the transverse area of the original vertical aerial vegetative stem SAM contributed to the derived vertical aerial vegetative stem SAM. During the second anisotomy exhibited by the vertical aerial vegetative stem SAM, 49.4% of the transverse area of the original vertical aerial vegetative stem SAM contributed to the derived vertical aerial vegetative stem SAM. Nonetheless, the shape of the two derived SAMs differed though they did not differ in size.

CONCLUSIONS: In D. digitatum, anisotomy results from an unequal bifurcation of the original SAM. This finding sheds light on plant body architecture evolution as well as plant organ (megaphyllous leaf) evolution.