Clematis 'Henryi' was one of the early hybrids created as part of an attempt to get the largest possible flowers on clematis.  In 1855 Isaac Anderson-Henry of Edinburgh crossed C. Patens and C. Lanuginosa and obtained the white-flowered 'Henryi'.
It is early-summer flowering with a second crop later in the year.  It doesn't grow too tall - about 3-4m, depending on how hard it is pruned.  The main attraction is the flower which is 15 to 18cm across with 8 sepals that barely overlap.  What adds to its interest is the contrast of brown-tipped stamens against the pure white of the flower.  Each sepal has elegant veining from tip to base.
In his book on clematis Christopher Lloyd states that it is a 'difficult' plant and he does not like it.  However, he does admit that it never grew for him.

photo by Pamela Clark