Book  (Oct 2001)  Page(s) 20.  
 
Clematis 'Sieboldii' was introduced in to the Leiden Botanic Garden in the Netherlands in 1837, by its director, Philipp von Siebold... C. 'Sieboldii' sported to produce another double-flowered cultivar, C. 'Plena', also known as C. florida 'Alba Plena'... [both were] introduced from the Netherlands into the British Isles in 1836 [which would be a year before the authors just wrote the former was introduced into the Leiden Botanic Garden???]

Book  (11 Oct 2000)  Page(s) 118.  

Book  (1816)  Page(s) 17.  
 
Classis 13. Polyandria....Ord. 6. Polygynia. Clematis var. Plena, Double flowered do. [ditto Japan clematis], Do. [Clematite de Japon] à fleurs doubles, Japan, Do. [Intro. by J.V. Thompson Esq. 1816], Reduit