Feb., 1919] New Varieties of Acer Rubriim L. 237 



Key to the Section Rubra of the Genus Acer in the United States. 



Leaves simple, palmately lobed; flowers in lateral umbels opening before the 

 leaves. 



1. Petals absent; ovulary tomentose; samaras 5 cm. long or longer 



.A. sacckarimim. 



1. Petals present; ovulary glabrous; samaras from 2-7.5 cm. long 2 



2. Samaras 4 cm. or more long; leaves large, firm, white tomentose beneath. 



A. drummondii. 



2. Samaras less than 4 cm. long; leaves pale or white glaucous underneath, 



sometimes pubescent on the veins 3. 



3. Leaves prevailingly 3-lobed, tips of lateral lobes curve forward; blade more 



or less pubescent underneath A. carolinianum. 



3. Leaves prevailingly though not conspicuously o-lobed, glabrous or glaucous 



beneath 4. 



4. Mature samaras shining, deep purple red, slender wings widely divergent 



more than 90°; leaves firm, veins and petioles dark red 



A. rubrum var rubra carpum. 



4. Mature samaras green or yellow sometimes tinged but never wholly red; 



leaves variable 5. 



5. Leaves and samaras green through all, stages of development; samara 



short and broad, wings straight, broad, contiguous or nearly so; seed 

 cavity broad and bulging A. nibrum var viride. 



5. Leaves and samaras more or less red; wings of samara always diverging, 



sometimes in curved, seed cavity slender 6. 



6. Samaras very small 2 cm. long strongly nerved A. stenocarpiim. 



6. Samaras more than 2 cm. long, not strongly nerved A. rubrum. 



EXPLANATION OF PLATES. 



Plate XII. Acer rubrum L. var. viride n. var. 



Fig. \. Mature samara. (Nat. size), collected May 18, 1917. 



Fig. 2. Young leaf. (Nat. size), showing size and condition of leaves at the time 



when the fruit is mature. May 18, 1917. 

 Fig. 3. Mature leaf. (Nat. size), collected August 9, 1917. 

 Fig. 4. Photograph of the tree. May 11, 1918. 



Pi ate XIII. Acer rubrum L. var. rubrocarpum n. var. 



Fig. L Mature leaf. (Nat. size), collected August 9, 1917. 



Fig. 2. Mature samara. (Nat. size), collected May IS, 1917. 



Fig. 3. Young leaf, (Nat. size), showing size and very tomatose under surface 



of leaf at the time when the fruit is mature. 

 Fig. 4. Photograph of the tree. May 11, 1918. 



