188 BOTANICAL GAZETTE [SEPTEMBER 
almost complete reversal of structural characteristics, but with 
far more conspicuous differences. 
Fig. 21 shows a section through the fruit (samara) of box elder 
at about the mid-level of the embryo. The walls of the seed vessel 
are very thick throughout, with a dense fibrous sclerotic lining of 
remarkable thickness. The fibrovascular strands are not very 
numerous, but some of them are quite large, with dense, centri- 
fugally massed pericycle. There is a subepidermal sclerotic zone 
of usually one layer of large thick-walled cells. The embryo is 
simply and symmetrically folded, and surrounded by a moderately 
thick, tanniniferous coat. 
Figs. 20 and 22 show sections of the fruit of A. platanoides. 
The wall is very thick at the base, but much thinner around the 
embryo. There is a fibrous sclerenchyma lining only near the base, 
and it is not thick or dense even here. Fibrovascular strands are 
numerous, but all are small. The subepidermal sclerotic zone 
is composed of small, thin-walled cells. The embryo is very irregu- 
larly folded, and covered with a thick, dense, tanniniferous coat. 
The box elder fruit is clearly better fitted to withstand unfavor- 
able weather conditions than is the fruit of the maple. 
Geological record of box elder 
The Acer group apparently made its first appearance in the 
Upper Cretaceous, and became widespread and diversified in 
species during the Eocene, in which fossil remains of various maples 
are abundant. However, there is recorded only a single instance of 
the mention of Acer Negundo in a fossil state, and that in the 
Miocene at Oeningen in Baden (Neues Jahrb. 1835. p. 55). But 
since this record has not been referred to by any recent authority, 
it may be set aside as of very doubtful value. Fossil leaves of the 
Negundo-like type are of rather frequent occurrence as far back as 
the Upper Cretaceous. LresQueREAUX in 1868 founded the genus 
Negundoides for a single species from the Cretaceous (Dakota 
Group) of Nebraska, but Pax, in his revision of Acer, reduced this 
genus to Negundo (Bot. Jahrb. 6:346. 1885). Under the genus 
Negundo the following fossil species have been described: EOCENE, 
N. europaeum Heer (Switzerland and Oeningen in Baden), .- 
