This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Erect or climbing shrubs. Leaves persistent or deciduous, entire, toothed or lobed. Flowers in large terminal corymbs or panicles, fertile small, sterile large and apetalous. Petals 4 or 5, valvate. Styles 4 or 5, free, or connate at the base. Fruit small, capsular; seeds numerous, minute. Between twenty and thirty species, chiefly Asiatic, a few from North America. The name is a compound of water, and a vase, from the cup-shaped fruit.
1. H. Hortensia (fig. 97). - The form originally introduced under this name is the most familiar in cultivation, and one of the most desirable of dwarf flowering shrubs, especially in the south, in the vicinity of the sea. In some varieties nearly or quite all the flowers are sterile, the lobes of the calyx being greatly expanded, and pink, white or blue, according to the nature of the soil; and in others only the outer flowers are sterile. The same curious transformation may be seen in the wild and cultivated varieties of the Guelder Rose. A native of Japan, introduced in 1790. The following forms, also Japanese, are with the foregoing all considered as varieties of one species; but, as varieties, many of them are very distinct and beautiful. H. Japonica roseo-dlba has the outer flowers only radiate, having either white or rosy toothed petals; H. Jap. caerulescens has bright blue ray-flowers. H. Otdksa, very near the common Hortensia, with nearly all the blue flowers sterile, and in very large panicles; H. stellata prolifera has pink flowers with several series of sepals in the sterile ones; H. Azisai, with ray or barren flowers on very long pedicels.
Fig. 97. Hydrangea Hortensia. (1/8 nat. size.)
The above varieties differ considerably in the size, pubescence and form of the foliage, but there is a general resemblance not easily mistaken. We must not omit to mention that there are some varieties prettily variegated in the foliage with red or yellow.
H. paniculata is a distinct species, in which the inflorescence is elongated, not flat or rounded. The variety grandiflora is a magnificent plant, in which all the flowers are sterile and pure white, forming a very large panicle. H. Thunbergii is a more slender species, with much smaller foliage, and flat cymes of blue flowers. The form in cultivation has a few of the outer flowers only barren, with rounded striated sepals. H. scan-dens, including H. petiolaris, is of climbing habit, and differs from all the preceding in the petals, which cohere at the tips and fall together. H. arborescens is an American species of larger stature, with inconspicuous greenish-white flowers, few of which are sterile and enlarged, H, quercifolia, another American species, has lobed leaves and terminal panicles of greenish white or pink flowers, in part barren.