34 JOURNAL OF THE ARNOLD ARBORETUM [vol. iv. 



Wil 



Of the remaining species 



China claims three (R. Fortunei Lindl., R. Championae Hook, and R. ovatum 

 Planch.) and the others are distributed through the territory dealt with 

 in this article. Three species only were known from China in 1870, 

 whereas today they run into hundreds. On the contrary there have been 

 no additions to the number known to Maximowicz from Japan and con- 

 tinental northeastern Asia. 



The Rhododendrons of northeastern Asia have in general been known 

 since a very long time and at least three species were known in Linnaeus* 

 time. Yet they have been slow in reaching the gardens of the west and 

 except R. dauricum Linn, and R. micranthum Turcz. have not yet found a 

 proper foot-hold in cultivation. They are all quite as hardy here as R. 

 catawbiense Michx. but like many other plants from high northern regions, 

 they are difficult to grow. Several of them fall victims nearly every year 

 to destructive spring frosts whilst in the case of others the right con- 

 ditions are hard to find. In England where choice among many species and 

 a plethora of hybrids obtains it is doubtful if the Rhododendrons of north- 

 eastern Asia will ever be considered of much general importance in 

 gardens. In eastern North America where the climate is more severe it 

 is different. For this part of the world R. brachycarpum D. Don and 

 R. Metternichii S. & Z. are most valuable plants and in the future will 

 probably play a highly important part in the evolution by hybridization 

 of a race of Rhododendrons suitable to the extreme climatic conditions 

 which obtain here. R. dauricum Linn., especially its var. mucronulatum 

 Maxim., is one of our best spring-flowering Rhododendrons and R. 

 micranthum Turcz. has the distinction of being the only species known 

 from China that is hardy in the Arnold Arboretum. 



KEY TO THE SUBGENERA 



Shrubs or trees with persistent, rarely deciduous, glabrous, lepidote or tomentose 

 leaves; stamens 5-20; ovary glabrous, lepidote or tomentose, never setose, 

 sometimes more than 5-celled; corolla 5, 7 or 9-lobed. . . .1. Eurhododendron. 



Shrubs with deciduous, non-lepidote leaves; corolla rotate to sub-rotate; stamens 5; 

 flowers solitary, rarely 2, from lateral, clustered buds II. Azaleastrum. 



Shrublets with deciduous, non-lepidote leaves; corolla rotate; stamens 10, flowers 1 

 or racemosely 2 or more from a terminal bud; bracts foliaceous. 



IIL Therorhodion. 



Subgen. I. EURHODODENDRON Exdl. 



Rhododendron subgen. Eurhododendron Endlicher, Gen. PL 759 (1839), 

 emended. 



Rhododendron sect, i Ponticum G. Don, (Jen. Syst. in. 843 (1834), including 



sections n, in and iv. 

 Rhododendron sect, in Eurhododendron De Candolle, Prodr. vn. 721 (1839), 



including ictions I, n, and iv. 



