A Theory of the Double Leaf-Trace. 85 
evidence as to progression or reduction drawn from the distribution 
among the alliances of the two extreme types. 
(a.)—Distribution of Types in Relation to Systematic 
Position of Plant. 
The tetrarch type is characteristic of the Leguminoseae, 
Rosaceae, Euphorbiaeae, Aceraceae, Balsaminaceae, Malvaceae* 
and occurs in Casuarina and some of the Polygonaceae and 
Amentiferae. 
Among the Sympetalae it is found in the Convolvulaceae, 
Sapotaceae and Cucurbitaceae. 
The diarch type is, with the exception of the orders just named, 
characteristic of the Sympetalae examined, and also of the Ranales, 
Rhoeadales, Urticales, Piperales, Umbellifloras, while it is found in 
some Polygonales and Centrospermae. 
It is obvious that the distribution of these types, as far as 
known, does not in itself point to any conclusion as to which is the 
more primitive. But this need not discourage one from drawing 
inferences from the distribution of the intermediate forms, nor 
from making use of other lines of evidence. 
The Ginkgo of the Angiosperms is Mirabilis Jalapa, a member 
of the Centrospermae, in which a tetrarch root is formed which 
reduces to diarch by suppression of the intercotyledonary poles. 
Mr. Hill informs me that the same thing occurs in M. divaricata, 
and also in Allionia, a plant belonging to the same order. 
Among the Ranales the curious intermediate form seen in 
Liriodendron tulipifera and Clematis Hendersoni is found, while 
Cassytha, 1 a parasitic member of the Lauraceae, is tetrarch. 
The likeness between Liriodendron tulipifera and Cycas 
Siamensis, which almost certainly shows a stage in reduction, is 
interesting, for it is well known that the Magnoliacea have 
frequently been compared with the Cycads on other grounds. 
Many of the Ranales possess two suspicious features (a) 
somewhat persistent laterals, (b) four phloem groups in the 
hypocotyl. These facts are suggestive, when we consider that 
of the neighbouring groups, the Rosales appear to be commonly 
tetrarch, while the Centrospermae show reduction from tetrarch 
to diarch, and many of the Polygonales are tetrarch. Moreover, 
the members of the Ranales most likely to prove interesting have 
not yet been obtained. 
1 Mirande. “ Le develop, et 1’ Anat. d. Cassythacees.” Ann. d. 
Sc. Nat. 9e Ser. Bot., tom. ii., 1905. 
