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Dahlgren’s system

Rolf Dahlgren was a Danish botanist who published an influential plant classification system in 1975. He used a two-dimensional graphic system called a Dahlgrenogram to display phylogenetic relationships among plant groups. Dahlgren's system was based primarily on morphological and chemical characteristics and divided angiosperms into 31 superorders within two subclasses. While comprehensive, it considered only flowering plants and did not classify below the family level. Subsequent molecular studies have revised placements of some families from Dahlgren's system.

Education◦Technology◦
 Rolf M.T. Dahlgren  1932-1987  Professor of botany in University of Copenhagen.  Published his original version of his system in 1975.  Most notable classification system in the resent history.
 Major interest= evolutionary characters within the angiosperms(class Magnoliopsida)  Used a two dimensional graphic system(Dahlgrenogram/bubble diagrams) to display his phylogenetic system.  Dahlgren himself was much interested in phylogeny.  Mostly morphological and phytochemical characters considered.  Used the “-iflorae” endings in his superorders.
Characters concidered..  Apocarpy,syncarpy & monocarpy conditions  Choripetaly,sympetaly & mono-aperaturate pollen types  Sclered idioblasts  Types of microsporogenesis  Bi and uni-tegmic ovules  Presence of raphids,silica bodies  Presence of various alkaloids & flavonoids etc.
Dahlgrenogram  Diagrams representing a cross section of a phylogenetic tree.  Groups are represented by bubbles of different sizes between which relationships are implied-though not clearly shown.  Much used to display results of broad surveys of variation of characters.eg- the types of plastids in the sieve tubes  More closely related to Bessey’s cactus-like diagram
Angiosperms (class Magnoliopsida/ class Angiospermae) subclass-Magnolideae subclass-Lillidae -represent dicotyledons -represent monocotyledons 24 super orders 07 super orders 80 orders 26 orders 346 families 92 families
Merits and demerits. Merits • Angiosperms are given a more agreeable rank. • No family of angiosperms is left unplaced. • Dahlgrenograms , -useful to show relationship of superorders,orders & families. -show relative numbers of each group. -useful to compare the distribution of various character states of angiosperms. • Superorder rank result in a realistic arrangement of families and orders.
Drawbacks • The system covers only flowering plants. • Does not proceed below the family level, not useful for arranging specimens in herbarium. • Based mostly on morphological & chemical characters. • Recent molecular studies justified the placement of some families.
References  Walters.D.R, Keil.D.J, Murrell Z.E, Vascular plant taxonomy, 5th edition ,page 105-109,Kendall/Hunt publishing company, Lowa,USA  Jones.S.B, Luchsinger.A.E, Plant systematics, 2nd edition,McGraw-Hill, Inc. NY .page 144-147  http://www.jstor.org/pss/3667872 (02.02.2012)